Drought and climate teleconnection and drought monitoring

Anteneh Z. Abiy , ... Wossenu Abtew , in Extreme Hydrology and Climate Variability, 2019

22.2.2 Agricultural drought

Atmospheric moisture deficit at critical times of crop development causes stunting of crop development attributed to soil water deficit. Driven by meteorological drought, the concept of agricultural drought considers the impact of meteorological drought on agrarian seasonal water demand creating soil water deficit. Hence, in addition to the spatial and temporal distribution of rainfall, agricultural drought deals with evapotranspiration. Evapotranspiration is a function of several parameters including water availability, land use land cover, and soil type. Most of these factors controlling evapotranspiration loss in an area have a spatial peculiarity. Hence, different regions with the similar rainfall pattern could have different susceptibility to agricultural drought.

A study of agricultural drought requires a comprehensive understanding of the soil hydrological behavior, plant physiology and development stage, and agricultural economics (Palmer, 1965). It incorporates the balance between soil water content, crop type, crop stage, and crop production practice. For example, an area could be in agricultural drought regarding a given crop type of high water demand at an early stage of development. The same condition could be suitable for low water requiring crop production activities; therefore, the agricultural drought would not be an issue (Lu et al., 2017).

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Volume 6

Chedlia Ben Ahmed , Bechir Ben Rouina , in Encyclopedia of Environmental Health (Second Edition), 2019

Distribution of Olive Plantations in Tunisia

Olive (Olea europaea L.) originating from the Eastern Mediterranean region of the Middle East is indigenous to the Mediterranean climate where long periods of soil water deficit are usually present during the dry seasons. Average world olive production is about 10 millions tonnes, 90% of which are processed to produce oil for human consumption. 95% of world olive orchards area is located in 10 countries of the Mediterranean basin. Olive plantation is mainly concentrated in Spain (167 millions plants), in Italy (125 millions trees), in Greece (120 millions trees), in Turkey (83 millions trees), in Tunisia (85 millions trees) and in Portugal (50 millions trees). Recently, thanks to health benefits of olive oil, many countries over the world, even those who don't have a tradition, are being interested in olive cultivation.

In Tunisia, olive tree is the most extended crop in this area, not only for its socio-economic role, nor for the health benefits of olive oil, but also for its great importance in the preservation of green area landscape in arid region, its limitation of desertification and the prevention of soil erosion and land degradation, and so its involvement in the maintenance of land durability. Furthermore, this species is characterized by its tolerance to contrasting environmental conditions distinguishing the arid climate of Mediterranean type of Tunisia.

At the social scale, olive cultivation provides between 20 and 30 millions working days yearly in Tunisia. Olive crop occupies 33% of cultivated lands in Tunisia with a total of 85 millions trees distributed all over the area from the North to the South. Among fruit plants, olive tree cultivation is extended to 80% of the cultivated area. Depending on the climatic conditions of each side, Chemlali olive is extended in the south, and the north landscape is characterized by the extension of the Chetoui cultivar. The biannual alternate bearing phenomenon characterizing this species leads to the irregularity of production. The intensity of this phenomenon is reinforced under the contrasting environmental conditions of arid climate in Tunisia. The alternate bearing is determined by intrinsic (genetic and metabolic characteristics) and extrinsic (soil type, climatic conditions, mineral status, …) factors that affect the biological cycle of olive tree.

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Runoff Generation in Badlands

Yolanda Cantón , ... Adolfo Calvo-Cases , in Badlands Dynamics in a Context of Global Change, 2018

4.1 Midlatitude Arid and Semiarid and Subtropical Badlands

The drier and warmer conditions that are predicted for most arid, semiarid and subtropical badlands (Fig. 5.5A and B ) will result in higher and extended periods of soil water deficit. That deficit may promote regoliths to display a higher density of desiccation cracks, in parallel to thicker regolith layers ( Calvo-Cases and Harvey, 1996). Hence, infiltration capacity after drying periods will increase in arid badlands, and most overland flow will probably reinfiltrate into the regolith through these cracks; overall, runoff rates will be reduced (Yair et al., 1980; Kuhn and Yair, 2004; Kuhn et al., 2004). On the other hand, it is expected an increase in extreme events for these regions (Fig. 5.5C) which will act in the opposite direction (Clarke and Rendell, 2006). In addition to precipitation and regolith properties, important changes are also expected in the main drivers that control flow connectivity and the hydrological behaviour at larger scales, such as biocrust cover, vegetation or land use. These changes will determine the way in which hillslope and channel network coupling changes during a rainfall event, and it may produce shifts in process dominance during both low and intense rainfall events, with meaningful effects on overall runoff production and underlying mechanisms.

Figure 5.5. Expected changes in mean annual temperature (A), duration of dry spells (B and C) frequency of high rainfall events (precipitation   >   20   mm) changes by 2100 with respect to 1986–2005 according to the RCP2.6; RCP4.5; RCP6.0; RCP8.5, reflecting an increase of +2.6, +4.5, +6.0, and +8.5   W   m−2 by the year 2100 relative to preindustrial values as determined by the Coupled Model Intercomparison Project (CMIP5).

Overall, Earth system models predict an increase in temperature (Fig. 5.5A) and a decline in total precipitation (Fig. 5.4A) for all seasons by the end of the century over most arid, semiarid and subtropical badlands such as most of the badlands within the Mediterranean region, South Africa, Cointzio badlands in Mexico or Victoria badlands in Australia (Figs 5.4 and 5.5). As runoff generation over many badlands is strongly dependent on rainfall (Nadal-Romero et al., 2008; Rodríguez-Caballero et al., 2014), climatic predictions point to a decrease in annual runoff rates. These predictions are in accordance with long-term climatic measurements over the Mediterranean region (Hurrell, 2007) and they perfectly match with previous studies that described a decrease in annual interrill erosion rates over Mediterranean badlands during the last decades. This decrease was found to be a consequence of reduced total annual rainfall and subsequent runoff occurrence (Clarke and Rendell, 2010). But, as we previously described, runoff connectivity between interrill areas and drainage network is controlled by the occurrence of runoff-effective rainfalls (Godfrey et al., 2008; Faulkner, 2008). Even though a decrease in interrill runoff can be expected under drier conditions, the predicted increase in the frequency and amount of extreme rainfalls will likely increase the occurrence of rainfalls with an intensity and magnitude sufficient for continuous runoff generation at the hillslope and catchment scale (e.g., rainfall events >20   mm; Fig. 5.5C). Moreover, the IPCC (2013) synthesis report points to a 'likely' decrease in the number of days with rain over these regions (Fig. 5.4B). This will probably affect regolith properties and morphology dynamics, likely limiting swelling and shrinking cycles, weathering and regolith development, and leading to the dominance of more persistent crust conditions and decreasing regoliths infiltration capacities (Nadal-Romero and Regüés, 2010). Hence, at the end, it is expected an increase in local runoff generation and water redistribution from hillslopes to drainage network.

Moreover, increased aridity and the expected longer dry periods (Fig. 5.5B) will lead a shift from well-developed biocrust communities to early stages of development (Reed et al., 2012; Maestre et al., 2013; Darrouzet-Nardi et al., 2015 among others) or by a replacement of biocrust by physically crusted bare regolith, in worst cases. This will likely reduce surface roughness and surface stability, with a subsequent increase in overland flow velocity and shear strength that may trigger rill formation, driving runoff water through preferential paths to drainage network and increasing hillslope connectivity (Rodríguez-Caballero et al., 2015, 2018). Besides these important changes in regolith and biocrust development, increased aridity will also exert a negative effect on vegetation coverage, leading to an increase in hillslope and catchment connectivity, especially on semiarid badlands. Also it is expected an extension of badland areas to the nearby prone surrounding areas or the development of badlands in other areas where currently climate conditions maintain a vegetation cover that impedes badland development (see Calvo-Cases et al., 1991b).

Different scenarios can be predicted in other semiarid badlands such as those from the Tarija region (Bolivia), Chambal badlands in India, or badlands in the central Huange within the Yellow River valley (China), where precipitation amount and the frequency of high rainfall events are expected to increase. These conditions may lead to an unequivocally increase in runoff at the different spatial scales.

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Land Reclamation and Restoration Strategies for Sustainable Development

Anwesha Banerjee , Suman Bera , in Modern Cartography Series, 2021

13.4.3 Monthly degree of dryness

Considering the month-wise degree of dryness, all districts have to gain low rainfall in each postmonsoonal month and premonsoonal month in an identical manner. Among all weather stations, only three stations of northern most Bengal like Darjeeling, Jalpaiguri, and Coochbehar, and two stations namely Uttar Dinajpur and Dakshin Dinajpur have received very low percent (Fig. 13.4) of rainfall in both pre- and postmonsoonal months. The cumulative deficiency of monthly rainfall creates dry weather which directly results in soil water deficit and subsequently crop water stress. Considering the degree of dryness, 11.92%, 22.41%, and 25.39% area situated in western Bengal are covered with moderately dry conditions in respective premonsoonal months like March, April, and May.

Figure 13.4

Figure 13.4. Percent of rainfall in (A) Postmonsoonal months and (B) Premonsoonal months.

The region with the wet condition is extended only in 22.89%, 22.1%, and 17.41% area of northern Bengal in aforesaid 3   months and this dryness is continued to monsoonal months. All the monsoonal months, i.e., June (32% area), July (17.65% area), August (16.06% area), and September (22.91% area) experience moderately dry conditions in the western and middle region of WB (Fig. 13.5). The maximum region faces near-normal conditions except for northern Bengal which further threats to cultivation. The situation of June is terrible to the farmers because the total western Bengal is covered with moderately dry conditions. But, the picture of postmonsoonal month is quite different due to its wetness. In all postmonsoonal months, though the percent of rainfall is low, a little amount of rainfall creates wet conditions in whole of southern Bengal and northern Bengal which is a little bit helpful to Rabi crop cultivation. But the increasing trend of temperature and potential evapotranspiration enhances the crop water demand which is only fulfilled by irrigation water supply.

Figure 13.5

Figure 13.5. Areal percentage of degree of dryness in the study area.

So, it is cleared that there is a strong relationship between rainfall and the degree of dryness. Considering the relation between rainfall and dryness, all districts have gained a low amount of rainfall in each postmonsoonal month and premonsoonal month in an identical manner. Among all weather stations, only three stations of northern most Bengal like Darjeeling, Jalpaiguri, and Coochbehar, and two stations namely Uttar Dinajpur and Dakshin Dinajpur, have received very low percent of rainfall in postmonsoonal months. For this reason, northern Bengal experiences dryness during the postmonsoonal months only. The cumulative deficiency of monthly rainfall from the postmonsoonal month, November, to premonsoonal month, April, creates accretive dry weather which directly results in soil water deficit and subsequently crop water stress. Drought as a natural hazard is different from water scarcity as it cannot be prevented by local water management (Van Loon and Lanen, 2013; Bandyopadhyay et al., 2020). From the above discussion, it is understood that in National level comparison, WB is good enough, but the dryness in local to the local, region to region is not so good. Except for the northern most Bengal, annual, seasonal, and monthly dryness also is observed in whole western Bengal, south Bengal, and Middle Bengal in different manner. Therefore, there are farmers' suggestions with their long-term experience about real dryness conditions during crop cultivation. No one can minimize the degree of dryness of any area because it is totally dependent on the climate of a particular area. But the impact of drought on agriculture and livelihood must be minimized by taking some initiatives by the local government or NGOs. So, the local Government should take early warning system to the common drought-prone area (DPA) or moderate dryness area; Government or NGOs should also take rising initiatives on the preparedness of plan to cope with drought at ground level; choosing of the crop should have changed from traditional to two or three droughts tolerating crop combination for better livelihood.

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Tools of the trade 3—mapping exposure and vulnerability

Chris Funk Phd , Shraddhanand Shukla Phd , in Drought Early Warning and Forecasting, 2020

6.2 Conclusion

While a full treatment of drought management and DRR is beyond the scope of this book, understanding how exposure and vulnerability can act to magnify the impact of a drought shock (Fig. 6.1) is an important part of effective drought early warning. Depending on the context, the same precipitation deficit may have very different impacts. In a wet region with soils with deep water-holding capacities, rainfall reductions may actually enhance crop yields, because plant growth in such regions is often limited by the amount of available radiation, not water. Under such conditions, sunny skies during the growing season may enhance plant growth. Conversely, in regions with high RefET or porous soils, water deficits may have serious impacts. There is a temporal aspect to such changes in vulnerability, since plant growth and evapotranspiration in many humid and semihumid regions may be water-limited at the beginning and end of the growing season but are typically radiation-limited in the middle of the growing season.

We have also briefly explored, how, ironically, growing prosperity does not necessarily mean an end to drought risk. At a global scale, increasing population and economic activity appear related to substantial increases in drought exposure and associated economic losses (Fig. 6.2). While countries such as Ethiopia and Kenya are experiencing rapid economic growth, the poorest people in these countries are still desperately poor, with annual incomes of 150–450 U.S. dollars (Fig. 6.9). As the income gap between the poor, middle, and upper classes increases in these countries (Fig. 6.10), the vulnerability of these poor households to drought-related food price spikes (Fig. 6.11) may actually be increasing too. At a global scale a recent report by the United Nations Food and Agricultural Organization (UN, 2018) finds a recent increase in the global number of undernourished people and underscores the need to develop enhanced climate resilience, early warning systems, and DRR.

As demonstrated by our brief analysis of exposure impacts in East Africa (Harrison et al., 2018) (Figs. 6.3–6.5), we are likely to see increased water stress as human populations continue to grow. Even in a world without climate change, drought risks are likely to increase. Effective DEWS will play a critical role in managing and mitigating these risks.

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Vegetation

Jean-Christophe Calvet , in Extreme Hydroclimatic Events and Multivariate Hazards in a Changing Environment, 2019

6.1.2 How are biophysical and biogeochemical processes related?

At the leaf level, transpiration and photosynthesis fluxes are connected because they share a common biological control mechanism called stomatal aperture (Bonan, 1995). Stomatas are small pores at the interface between the leaf tissues and the ambient air. Their aperture responds to bioclimatic variables such as leaf temperature, leaf-to-air saturation deficit, and the solar radiation reaching the leaf. High stomatal conductance triggers a CO2 flux from the air to the leaf tissues, thus feeding photosynthesis and plant growth. At the same time, water is lost through transpiration, especially at large values of the leaf-to-air saturation deficit. Stomatal aperture is the result of a compromise between photosynthesis and the need for the plant to avoid excessive soil water deficit values that could trigger early senescence. Stomatal aperture can change within minutes. The above-ground and below-ground vegetation architecture is an overarching characteristic of land surfaces. Key variables of the soil–plant system are leaf area index (LAI), canopy height, plant above-ground biomass, rooting depth and root density profiles, and litter mass.

LAI is expressed in units of m2  m−2 and is defined as the one-sided leaf area per unit ground horizontal surface area. LAI is particularly important for water fluxes since it governs transpiration, the energy budget at the soil surface, and soil evaporation. It influences rain interception. While intercepted rain suppresses leaf transpiration to some extent (Tuzet et al., 2017), part of the intercepted rain can be directly evaporated back to the atmosphere. In the case of forests, the biomass of the trunks and branches can store heat and can provide energy to intercepted rain evaporation (e.g., Cisneros Vaca et al., 2018). This process can also impact snow accumulation (Roth and Nolin, 2017). Tree height is key, as tall canopies increase surface roughness, resulting in enhanced heat exchanges with the atmosphere. Rooting depth is also a key variable for water fluxes as it governs soil water uptake and the contribution of groundwater to plant transpiration (Bevan et al., 2014). Finally, litter is often present in forests and grasslands. Litter impacts the energy budget at the soil surface and can intercept rain. Soil organic content is a parameter in state-of-the-art LSMs, related to porosity, soil hydraulic conductivity, and soil water holding capacity.

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HYDROLOGY | Impacts of Forest Management on Streamflow

L.A. Bruijnzeel , R.A. Vertessy , in Encyclopedia of Forest Sciences, 2004

Effects on Forest Water Use and Catchment Water Yield

Whilst the effect of thinning on interception and net precipitation is thus seen to be rather limited, effects on soil water (and ultimately streamflow) are likely to be smaller still. Opening up of a stand not only enhances the penetration of radiation to the understory vegetation and the forest floor (thereby enhancing evaporation), but also the remaining vegetation will start to compete for the extra moisture supplied by the initially increased throughfall. The magnitude and the duration of such effects will differ between locations, depending on the vigor of overstory and understory vegetation, climatic conditions (including slope exposure), and the configuration of the cutting, as shown by the examples below.

No changes were detected in the streamflow from a deciduous hardwood forest catchment of southeasterly exposure in the southeastern USA (Coweeta) after a selective logging operation had removed 27% of the basal area, whereas only a 4.3% rise in flows was observed after a 53% selective cut. Removing the entire understory (representing 22% of forest basal area) from a catchment of northwesterly exposure in the same area produced an equally modest change. Typically, the moisture gained by removing one component of the forest is rapidly taken up by others. In coastal Douglas-fir forest in western Canada soil water deficits developing during the summer have been shown to be very similar below dense, unthinned stands with little to no undergrowth, and thinned stands with a well-developed understory. After removal of the undergrowth, soil water uptake by the trees increased by 30–50% and the overall effect of the removal on soil water content was insignificant. In an experiment involving two 40-year-old Scots pine plantations of similar tree height but with a more than fivefold difference in stocking in the UK, tree water uptake (transpiration) in the widely spaced plantation was about two-thirds of that of the denser stand. However, relative transpiration rates per tree were more than three times higher in the thinned plot, and intermediate in magnitude between the relative increases in average water-conducting area (so-called sapwood) per tree (2.9 times) and leaf area per tree (4.2 times), compared to the unthinned stand. Therefore, although water use by the thinned forest had not reached prethinning levels yet, the large increases in leaf and sapwood areas of the remaining trees could be seen as representing a tendency towards complete re-equilibration following a set of physiological relationships aimed at maximum site utilization. Finally, in an extreme case from South Africa any positive effects on streamflow of three rounds of thinning (45%, 35%, and 50% after 3, 5, and 8 years) Eucalyptus grandis plantations were masked entirely by the continued reduction in flows resulting from the overall vigorous growth (and thus water uptake) of the trees. The message from these examples is a clear one: thinning has to be rather drastic before a marked effect on streamflows can be expected.

Selection logging in tropical rainforest does not produce measurable effects on streamflow for harvesting volumes up to 20   m3  ha−1 but the much higher logging intensities practised in the rich forests of Southeast Asia have a marked effect. Typical increases in annual water yield under 'average' rainfall conditions (c. 2000   mm year−1) and harvesting intensities (33–40% of the commercial stocking) amount to 100–150   mm but larger increases are possible where harvesting is more intense and disturbance of the soil more widespread. The effect usually disappears within a few years as logging gaps become recolonized (Figure 3) although compacted surfaces like tractor tracks, roads, and log landings continue to be sources of enhanced runoff for much longer (decades).

Figure 3. Soil moisture content in the top 70   cm of soil below undisturbed tropical rain forest, 6-year-old secondary growth, and in a narrow (10×50   m) and a large (50×50   m) clearing in lowland Costa Rica during two consecutive dry seasons. Reproduced with permission from Parker GG (1985) The Effect of Disturbance on Water and Solute Budgets of Hillslope Tropical Rainforest in Northeastern Costa Rica. PhD thesis, University of Georgia, Athens, GA.

Apart from the degree of cutting, the configuration of the resulting gaps also has an influence. During the first dry season after the creation of differently sized gaps in tropical rainforest in Costa Rica, soil water reserves were depleted most rapidly under undisturbed forest, followed by 6-year-old regrowth, pioneers in an elongated narrow gap, and pioneer vegetation in the center of a large square gap (Figure 3). Only 1 year later, however, soil water depletion in the smaller gap already resembled that of the 7-year-old vegetation, whereas that for the larger gap had increased considerably as well. The higher water uptake by the vegetation in the smaller gap compared to that in the larger gap reflects the more rapid recolonization of smaller gaps as well as additional uptake by trees from the surrounding forest sending their roots into the gap (Figure 3).

The influence of the configuration of the cutting on the magnitude and duration of any increases in streamflow has been investigated in some detail. In the eastern USA the removal of 24% of the basal area from catchment LR 2 at Leading Ridge (Pennsylvania) caused a nearly twofold larger increase in flow than cutting 33% of the forest on catchment HB 4 at Hubbard Brook (New Hampshire) or catchment FEF 2 at Fernow Experimental Forest (West Virginia) (see Figure 4). The cutting at Leading Ridge consisted of a single block on the lowest portion of the catchment, whereas the cutting at Hubbard Brook took the form of a series of strips situated halfway up the catchment, and that at Fernow involved harvesting trees from all over the catchment. Therefore, increases in streamflow associated with strip cutting are smaller than for single blocks. This is in agreement with the finding of increased water uptake by surrounding trees upon opening up of the canopy (Figure 3) and the limited effect on rainfall interception by thinning described earlier.

Figure 4. First-year increases in water yield in response to forest cutting in the northeastern USA. Reproduced with permission Hornbeck JW, Adams MB, Corbett ES, Verry ES, and Lynch JA (1993) Long-term impacts of forest treatments on water yield: a summary for northeastern USA Journal of Hydrology 150: 323–344. MEF, Marcell Experimental Forest, MN; FEF, Fernow, WV; LR, Leading Ridge, PA; HB, Hubbard Brook, NH.

No significant differences in streamflow increases were found between the cutting of the upper half of a catchment (such as at catchment 7 at Fernow; FEF 7 in Figure 4) or the lower half (catchment FEF 6 in Figure 4). Likewise, removal of the vegetation around watercourses in areas with a high rainfall surplus did not produce increases in water yield above those associated with the removal of an equal area of forest elsewhere in the catchment. However, where forest evaporation consumes a much larger proportion of the rainfall and where the terrain is more gently sloping than in the examples shown in Figure 4, the effect of cutting trees in the lower third to half of a catchment may well be rather more pronounced than that of cutting the upper third to half. Under such subhumid conditions, water uptake by trees having ready access to groundwater is usually higher than that of trees further up the slopes. High water tables are typically associated with the areas around streams (the riparian zone), footslopes and depressions in the landscape; these, in turn, are mostly found in the lower parts of catchments.

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Ecohydrology and the Critical Zone: Processes and Patterns Across Scales

Georgianne Moore , ... Greg Barron-Gafford , in Developments in Earth Surface Processes, 2015

8.3.1.2 Energy-Limited Regions

Many regions of the world are not considered water-limited and the dominant ecohydrological processes differ considerably (Fig. 8.1 ). Precipitation in wet environments tends to exceed ET rates, at least for much of the year, leading to high rates of percolation, runoff, and groundwater recharge. This poses a different set of dominant processes for such systems, which in ecohydrological terms are referred to as "energy-limited." In the context of the Critical Zone, "energy-limited" environments are typified by saturated or near-saturated soils, unrestricted plant-available water, or very low frequency of soil water deficit. Many of these regions harbor highly productive, yet highly biodiverse plant communities. Ultimately, much of the Sun's energy is captured and cycled within the biota. Nutrients are also tightly cycled within the biota. Conventional wisdom dictates that wet tropical forests retain most available nutrients within aboveground biomass ( Proctor, 1987), although stochastic characteristics within these systems (e.g., aboveground and belowground heterogeneity) have hindered efforts to quantify nutrient budgets (Proctor, 2005). Critical Zone processes that exert strong controls on the biota in these environments include decomposition, leaching, and nutrient cycling. On a global scale, carbon cycling and land conversion are issues of fundamental concern in these regions.

In energy-limited environments, net radiation and atmospheric vapor pressure deficit explain much of the variation in plant ET (Loescher et al., 2005; Fisher et al., 2009). In regions with more than 2000   mm rainfall, annual ET approaches an energy-driven limit of approximately 1600   mm constrained by high humidity and cloud cover (Zhang et al., 2001; Bonell and Bruijnzeel, 2005). Rarely, if ever, do plants in these environments experience water deficits sufficient to suppress plant transpiration. However, additional factors occur such as waterlogging stress (such as in wetlands) and continuously wet surfaces (such as in tropical rainforests) that may stress plants to lower transpiration rates depending on species adaptations. Furthermore, plants can affect the depth to the water table, which in turn exerts controls on soil microbial activity and hydrologic fluxes. Thus, the presence of a water table presents challenges for characterizing the water balance in energy-limited systems compared with water-limited systems that generally do not root into the water table.

Evaporation of intercepted rainfall becomes a significant portion of the water balance in high rainfall regions (see Fig. 8.1, Holwerda et al., 2012), leading to irregular throughfall distribution and soil wetting (Loescher et al., 2002b). Intercepted rainfall or fog can also be absorbed directly by leaves via foliar uptake (Limm et al., 2009), which in some systems impacts the water balance. Where fog is prevalent and canopies are perpetually wet, cloud forests occur (Eugster et al., 2006). At the extreme, energy limitations lead to very slow-growing, dwarf vegetation (Bonell and Bruijnzeel, 2005).

Further, vegetation in high-rainfall regions contributes to "precipitation recycling." For example, one third of the precipitation that falls over the Amazon basin is supplied by recycled ET, which is further facilitated by the deep-rooted trees (e.g., Trenberth, 1999). Hence, ET has important influence on precipitation in tropical forests through land–atmosphere interactions. Anthropogenic effects of land cover change can also have profound impacts on the hydrologic cycle in these regions (Wohl et al., 2012). Additionally, climate change is predicted to modify the interaction between vegetation and the water cycle in humid regions by increasing atmospheric-moisture storage and altering the amount and timing of rainfall.

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Snow, Ice, and the Biosphere

Terry V. Callaghan , Margareta Johansson , in Snow and Ice-Related Hazards, Risks, and Disasters, 2015

5.3.2 Modification of Nonwinter Habitat: Duration of the Snow-Free and Ice-Free Periods

5.3.2.1 Snow

The duration of snow is a critical determinant of the length of the growing season for plants, and thus, the length of the season when these primary producers can be accessed by herbivores and other dependents. The length of the snow-free period is variable from year to year and has increased in the Arctic by about 13   days between 1972 and 2008 due to global warming (Callaghan et al., 2011; Derksen and Brown, 2012). The length of the snow-free period generally increases from high polar latitudes to temperate latitudes with the exception that high Arctic and Antarctic polar deserts can be very dry with little snow cover. Snow cover also varies with topography, accumulating in late-lying snow bed habitats and being blown away from exposed ridges or fell fields (Figure 5.2).

The patterns of snow accumulation on the landscape are probably the major determinant of plant species and community distributions (Evans et al., 1989 ). At one extreme, fell field plants must be adapted to withstand low winter temperatures, high winds, and soil water deficits when they are exposed to winter climate. At another extreme, where snow persists into late summer in depressions with deep snow accumulation, plants must be able to complete their seasonal cycles of leaf production and reproduction within a very short time. In contrast, the impact of snow on animals depends on their abilities to migrate. Arctic residents such as musk ox and lemmings experience the complete snow climate, although some animals such as brown bear and ground squirrels hibernate. Short-distance migrants such as reindeer, arctic fox, and ptarmigan are affected by snow conditions within the Arctic and in the winter grazing lands in the forests to the south. Animal species that migrate over long distances to the Arctic in spring (e.g., waders and geese) are influenced by spring snow conditions when they arrive in the Arctic.

Overall, the duration of the growing season affects vegetation production, herbivore population growth, and the success of predators. On Ellesmere Island, short growing seasons reduced plant productivity, decreased populations of the herbivores musk ox and Arctic hare, and reduced the population size of their predator, the wolf (Mech, 2004). The length of the growing season also affects biogeochemistry. For each day earlier, modeled productivity of vegetation over the panArctic Region increases with a carbon drawdown of 9.5   gC/m2 (Euskirchen et al., 2006) when moisture is nonlimiting.

5.3.2.2 Lake and River Ice

The duration of lake and river ice in the North has decreased between 1846 and 1995, and this trend is predicted to continue in the future (Prowse et al., 2011). This might lead to higher primary production in lakes and rivers as more light is available for photosynthesis and more inputs of nutrient from the catchments is expected under warmer conditions (Vincent et al., 2011). It is also possible that the increasing open water areas can attract more aquatic bird populations (Vincent et al., 2009) and could possibly also favor less-cold-tolerant plants, fish, and other organisms that are likely to invade from the south. On the contrary, these warmer conditions may pose a threat to cold-adapted specialists such as Arctic char (Power et al., 2008) and could even drive some northern species to extinction (Sharma et al., 2007). Also, while increases in photosynthetically active radiation, resulting from earlier lake ice melt, enhance productivity, the removal of ice will give greater exposure of sensitive organisms to harmful UV-B radiation, particularly in temperate alpine lakes with high solar angles and thinner ozone layers.

5.3.2.3 Sea Ice

During 2004–2014, the minimum sea-ice extent in the Arctic Ocean has broken new minimum records several times, especially in 2007 and 2012, and it is now likely that the Arctic will be free of ice during the summer within 30 to 40 years (Meier et al., 2011; Jeffries et al., 2013). The sea ice is a unique ecosystem and home to a range of marine mammals such as the polar bear, walrus, and seals, and for different communities of other biota that live within or in association with the ice (Figure 5.9). Ice margins and gaps in the sea ice (polynias) are sites of high productivity where birds and mammals congregate to feed (Meier et al., 2011). Algal blooms follow the retreating ice edge using new nutrient pools when the polar night is over.

FIGURE 5.9. The Arctic sea ice margin is an area of high productivity on which complex food webs depend. The schematic shows the sea-ice community during the spring ice retreat and melt period.

 Source: drawn by Audun Igesund, Norwegian Polar Institute and published by Meier et al. (2011).

Marine ecosystems have been affected by the decreasing sea ice (Vincent et al., 2011). For example, satellite observations indicate an increase of phytoplankton biomass as a result of the shorter ice duration (Arrigo et al., 2008). Different whale species will most likely be affected in different ways depending on their specialization and flexibility of diet. Migrating whales are, for example, likely to expand their ranges and periods in northern waters (Vincent et al., 2011). Another example of a species that has been affected is the polar bear. A decline in condition and reproductive success of polar bears was reported from western Hudson Bay in Canada (Regehr et al., 2007). Polar bears are very sensitive to changes in sea-ice conditions and drastic declines of up to two-thirds of their population are predicted for the future (e.g., Durner et al., 2009).

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Snow, ice, and the biosphere

Terry V. Callaghan , Margareta Johansson , in Snow and Ice-Related Hazards, Risks, and Disasters (Second Edition), 2021

5.4.2 Modification of nonwinter habitat

5.4.2.1 Duration of the snow-free period

The duration of snow is a critical determinant of the length of the growing season for plants and thus the length of the season when these primary producers can be accessed by herbivores and other dependents such as pollinating insects. The length of the snow-free period is variable from year to year and increased in the Arctic by 2–4   days per decade with the largest negative trends occurring at high latitudes and elevations, which is consistent with polar amplification of warming and enhanced albedo feedbacks (Brown et al., 2017). The length of the snow-free period generally increases from high polar latitudes to temperate latitudes with the exception that high Arctic and Antarctic polar deserts can be very dry with little snow cover. Snow cover also varies with topography, accumulating in late-lying snow bed habitats and being blown away from exposed ridges or fell fields (Fig. 5.3).

The patterns of snow duration on the landscape are probably the major determinant of plant species and community distributions (Evans et al., 1989). At one extreme, fell field plants must be adapted to withstand low winter temperatures, high winds, and soil-water deficits when they are exposed to winter climate. At another extreme, where snow persists into late summer in depressions with deep snow accumulation, plants must be able to complete their seasonal cycles of leaf production and reproduction within a very short time. In contrast, the impact of snow on animals depends on their abilities to migrate. Arctic residents such as musk ox and lemmings experience the complete snow climate, although some animals such as brown bear and ground squirrel hibernate. Short-distance migrants such as reindeer, Arctic fox, and ptarmigan are affected by snow conditions within the Arctic, while reindeer are also affected by snow conditions in their winter grazing lands in the forests to the south. Animal species that migrate over long distances to the Arctic in spring (e.g., waders and geese) are influenced by spring snow conditions and an associated increase in productivity when they arrive in the Arctic (Madsen et al., 2011).

Overall, the duration of the growing season affects vegetation production, herbivore population growth, and the success of predators. For each day earlier, modeled productivity of vegetation over the pan-Arctic region increases with a carbon drawdown of 9.5   gC/m2 (Euskirchen et al., 2006) when moisture is nonlimiting. However, this estimate omits the response of soil respiration and permafrost thaw (as discussed later) to increased temperatures and the current and significant browning trend with plant damage through icing events and forest fires.

5.4.2.2 The timing of the spring thaw

In addition to the duration of the snow-free period, the timing of the melt and snow-on events is also of great biological importance (e.g., Wipf and Rixen, 2008).

In the Arctic, an asynchrony occurs between the timing of the snow melt and solar elevation that determines the amount of energy available for photosynthesis (Lewis and Callaghan, 1976). Thus, plants develop a photosynthetic canopy when the solar angle is declining and incoming radiation is not optimal. In contrast, when plants senesce in autumn, the solar angle is very low. During the current climate warming, the snow-free season is becoming earlier and plants are experiencing a greater synchrony and optimization of incoming radiation. This has an impact on biogeochemical cycles in that carbon sequestration increases. In contrast, a delay of autumn when solar angles are low is not expected to lead to increased plant productivity (Callaghan et al., 2005), but an increase in soil microorganism activity due to warmer soils with a result of increasing emissions of CO2. Surprisingly, snow manipulation experiments have shown that the generalizations of the effects of changing timing of the snow-free period are not always validated. Although considerable increases in snow depth would be expected to lead to delayed spring melt, Walker et al. (1999) showed that experimentally increased snow depth had little effect on the timing of snow melt when melt was fast. Also, although an earlier melt would be expected to increase the growing season length for plant production, Starr et al. (2008) showed that an earlier melt might instead result in earlier senescence and no net increase in plant growth.

Early-season snow cover also affects animals, both residents and migrants. Long-term observations show that a 10% decrease in spring snow cover advances egg laying by 5–6   days in one goose species and increases breeding success by 20% (Madsen et al., 2007). Similarly, population size of musk oxen in Northeast Greenland increases as the length of the snow-free period increases and early-spring snow cover decreases (Forchhammer et al., 2008).

The timing of spring thaw has major effects on the phenology (timing of events) in both plants (Wipf and Rixen, 2008) and animals. As spring thaw is becoming earlier, many phenological events are also becoming earlier. The significant warming and greening of 37% of the Arctic estimated from satellite images since the early 1980s (Xu et al., 2013) have been associated with a considerable seasonality or phenological advancement of the plant-growing season. For example, flowering occurring up to 3   weeks earlier has been reported in Northeast Greenland (Høye et al., 2007). Across six plant species monitored at Zackenberg, the timing of flowering was closely associated with spring snow cover in the year of flowering, with an average 2.5-day advance following a 10% reduction in snow cover on June 10 (Forchhammer et al., 2011). However, no such responses were found in the growth and reproductive output of the same plants. Furthermore, when climate effects such as the timing of snow thaw are considered across trophic levels, such as consumer-resource interactions, a phenological response of a plant species may have significant effects on its herbivore consumer, as reported in a reindeer population in West Greenland (Post and Forchhammer, 2008), but not necessarily on its own performance. In contrast, reindeer calf production in Finland increased by about one calf per 100 females for each day of earlier snow melt (Turunen et al., 2009). Although Xu et al. (2013) showed an earlier start to the growing season and associated this with greening of the vegetation between 1982 and 1998, Bhatt et al. (2017) and Epstein et al. (2017) demonstrated that an almost similar area of the Arctic experienced browning during the period 1999–2015. Thus, the increased duration and earlier timing of the snow-free period do not necessarily enhance primary production because of numerous other factors such as rain-on-snow events and moisture deficit in summer (as discussed later).

5.4.2.3 Meltwater supply

Snow stores winter precipitation in solid form and releases liquid water in spring. This liquid water is a particularly important source of soil moisture for plants in continental areas where summer precipitation is low. In the coastal tundra of Alaska, a longer growing season has not been associated with greening of the vegetation as expected (Gamon et al., 2013). The explanation is soil moisture deficits in the late growing season. Farther south, the growth of at least some boreal evergreen species depends on moisture from melting snow (Yarie, 2008). As the moisture regime in much of the boreal region is only marginally suitable for forest growth, reduced snow cover has probably led to the forest decline and loss of forest productivity recorded by Lloyd and Bunn (2007) and Goetz et al. (2005). This forest response leads to a spiral of events that lead to further forest destruction: Trees weakened by lack of soil moisture are more prone to fire and damage from insect pests such as the European spruce engraver beetle (Bakke, 1989), the North American engraver beetle (McCullough et al., 1998), and the Siberian silk moth (Kharuk et al., 2003). Changes in soil moisture supply from melting snow and its multiple effects are expected to lead, in some areas, to a change from needle-leaf evergreen trees to deciduous trees that are commercially less valuable (Juday, 2009).

Meltwater from snow has major effects on ground waterlogging during spring, and this will affect emissions of carbon (both amount and species—CO2 or CH4) and N2O (Callaghan et al., 2011). Evidence also exists that the magnitude of tundra fires is increasing (Mack et al., 2011). This has been associated with increases in thunder storms and lightning strikes, but it is not known if changing snow conditions are leading to drier and more fire-vulnerable tundra.

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