Links Between Climate, Hydrological Processes and Soil Development

Climate significantly impacts many parts within the earth system and leads to spatial and temporal variations in the processes involved. Climatic influences on the distribution of precipitation and evaporation determine hydrological and soil development process for different regions. The various processes act at different timescales ranging from a few moments of intense rainfall, to soil development over thousands of years. This essay will discuss some of the processes involved across the climate-hydrology-soil systems, the timescales they operate over and the implications on, and resulting from, climate change.

The climate-hydrology system works on a geographically rapid timescale, with evaporated waters remaining in the atmosphere for an average of just 10 days (Holden, 2008), illustrating the dynamic flux of water into and out of the atmosphere, and the impacts on other system processes. 87% of this evaporation comes from the ocean and is evaporated under the influence of solar radiation energy, with other climatic factors, such as wind velocity and humidity, impacting the rate. Figure 1 shows the distribution of water vapour in the atmosphere, with large amounts produced from oceans is transported over land by atmospheric circulation systems (Lal, 2016), before rising and undertaking collision-coalescence to form rain droplets, resulting in a complex distribution of precipitation. High latitude areas experience low precipitation rates due to the low vapour content of cold air, whilst the equatorial maximum for rainfall, slightly displaced into the Northern hemisphere, is as a result of converging trade wind systems, where its strong easterly component blows moist air from ocean areas and the winds influence convergence zones (Lal, 2016).   

Precipitation that reaches the ground either infiltrates in or flows over it, impacting many processes in the hydrological cycle and soil development. A soil’s infiltration capacity decreases throughout a rainfall event until a stabilisation point is reached and the water that infiltrates is then redistributed throughout the soil via percolation. The significance of the changing infiltration capacity is that rainfall intensity may exceed the infiltration rate and result in infiltration-excess overland flow as surface storage occurs (Smithson, et al., 2008). Rainfall intensity impacts the amount of water infiltrating the soil, and is dependent on the combined kinetic energy of the rain drops, which is controlled by droplet size and updraw strengths that determine the terminal velocity (Smithson, et al., 2008). The droplet size is determined by the rate of coalescence and the size of the condensation nuclei (Fraser, n.d.), and can also impact soil erosion. The effects of atmospheric turbulence, chaotic changes in pressure and flow velocity that occur for just 0.1 seconds (Smithson, et al., 2008), can enhance development of rainwater in clouds as turbulence increases the frequency of binary collisions (Seifert & Onishi, 2016), increasing the stochastic growth of large droplets by gathering water from all smaller droplets through collision-coalescence (Berry, 1967).  

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Rainfall intensity increases with climate change as global warming increases evaporation rates from both ocean and land, resulting in roughly a 7% increase in water vapour in the atmosphere per 1֯ C of warming (Hartmann, et al., 2013). A warmer atmosphere can hold a greater concentration of water vapour while also being more turbulent, increasing the frequency of collision events between droplets and thus rainfall intensity. Intense rainfall events may only last a few hours but can greatly impact the hydrological cycle and local areas may experience flash flooding if the intensity exceeds soil infiltration rates.  

The many processes within the climate-hydrology system can impact soil development in various ways including weathering and influencing soil respiration. These processes are constantly changing the soil properties over long periods of time, with the formation of a 30cm thick layer of soil taking between 1000-10,000 years (Hall, et al., 1992). Weathering, the in-situ breakdown and transformation of parent materials into detrital sediment (Holden, 2008), is a key component in soil development, and is influenced by climate and hydrological processes. Climate determines the conditions that soils develop in, and precipitation and evaporation rates and these factors influence the amount of water that percolates into the soil, which is a stimulant in many weathering processes. Weathering rates therefore vary across different climates and this is shown by using data collected from across the world in different climatic regions, averaged from a 20-year period and varying underlying geologies (Figure 2). As the temperature and liquid water availability rises across the climatic regions, the weathering rates increase to a maximum in tropical regions. The main input to form soils is mineral particulates of the parent material released by weathering, with by-products leached out of the soil (Holden, 2008). Water is the main agent of chemical weathering, with H+ and OH– ions in water reacting with minerals, producing new, usually physically weaker, minerals that are easier to transport (Lal, 2016). Fluctuating temperatures can play a part in physical weathering mechanisms, with expansion and contraction of rocks leading to breakdown under thermal pressure (exfoliation), or freeze-thaw action when water in pores and cracks expands by 10% upon freezing, exerting a force (Holden, 2008).        

With temperatures increasing globally, more so in the higher latitudes, there will be a loss of ice as glaciers and ice sheets experience greater levels of ablation, and reduced snow accumulation. Areas once covered by glaciers and ice sheets will become exposed, and the rocks below subject to subaerial processes. Warming will also thaw permafrost, increasing the depth of the soil active layer, allowing more weathering and other pedogenesis processes to occur (Hall, et al., 1992). Weathering rates will increase with higher temperatures, as with every 10 °C temperature rise, chemical reaction speeds double and physical weathering mechanisms can be affected, particularly if temperatures vary (Holden, 2008). Increased weathering rates can lead to a negative feedback loop. Specific areas may be impacted differently by warming, with Antarctica experiencing less freeze-thaw of rocks during the main spring to autumn period with higher temperatures and more incident rainfall. However other polar regions will see an increase in soil development rates, with the previous temperature and liquid water availability constraints being eradicated(Hall, et al., 1992).            

High levels of precipitation can lead to distinctive soil profiles, especially those found in waterlogged conditions where pore spaces between soil particles are filled with water not air, so reduction processes occur instead of oxidation. Climate contributes to creating these waterlogged conditions, where high precipitation and low evaporation rates lead to the rising of the water table as water infiltrates the soil until full saturation, as seen in areas of upland Britain (Smithson, et al., 2008). Peat soils, formed in waterlogged conditions, are composed of accumulated plant material remains that have not fully decomposed, as no aerobic decomposer organisms can survive in the anoxic conditions created. The amount and type of peat is influenced by the hydrological cycle, with the ionic quality of the precipitation and run-off affecting the ecosystem (Lal, 2016). The peat type depends on the plants that inhabited the area before and the plant types are dependent on hydrology and water chemistry, with precipitation being the main nutrient input for the vegetation (Smithson, et al., 2008). The timescale for peat formation and development is very long, as it takes time for vegetation to grow, die and then accumulate over 100s of years to form peat. Peatlands can impact the climate and hydrological systems, as they act as large stores of both carbon and water, globally holding 10% of the world’s drinking water and carbon levels equalling the total CO2 held in the atmosphere (Lal, 2016, p. 1668). These stores are at threatening of being reduced, as increased warming of the planet causes peatland areas to dry out and degrade, further impacting the climate and hydrological cycle. This degradation leads to the releasing of large amounts of dissolved organic carbon (DOC) into the water system, impacting downstream rivers drainage (Lal, 2016). The drying out of peatland will lead to enhanced respiration and lower rates of photosynthesis, so less carbon uptake (Strack, et al., 2006). A study of the Rzecin peatland in Poland (Rastogi, et al., 2019) examined the possible effects of future climate conditions on peatland, and the behaviours of important peatland plant species. The study suggested that warming and reduced precipitation may lead to a change in vegetation types of peatland due to moisture stress, interfering with stored carbon in the peatland possibly to an extent that peatlands convert to carbon sources, and lead to a positive feedback loop enhancing climate change. 

Soil erosion, the removal and transportation elsewhere of soil particles from the ground, can be caused by water, wind and gravity. This natural process, although often altered and sped up by human interference, is critical in forming soils from parent material and transferring particles. Soil erosion by water can occur in many ways depending on the topography and water flow. In upland areas, raindrops falling onto the soil with high speeds have enough momentum to compact the soil, whilst ejecting some sediment upon impact to be transported downhill, in a process called sheet erosion (Lal, 2016). This sheet erosion can be impacted by the size of the droplets formed in the atmosphere, as aforementioned. Surface run-off of can lead to soil erosion, particularly in large fields and watersheds where rills, formed by a large concentration of run-off, act as erosion channels, with the water flowing through and picking up loose soil, carrying it in suspension or as bedload that can further erode the soil surface (Lal, 2016). Soil erosion rates can be altered by changes in climate, as seen in North America in 1930. Crop failures, following a few drier than usual years, led to unprotected soils drying out and losing structural stability, so far more susceptible to being detached from the land surface by wind (Smithson, et al., 2008).

Soil respiration represents the combined respiration of roots and soil micro and macro-organisms, often partaking in litter decomposition, and is the second largest flux of carbon, primarily as CO2, to the atmosphere. The microbial decomposition of organic debris is essential for forming soil organic matter from the debris’ biomass (Hall, et al., 1992). There are many critical factors that affect soil respiration and the rate of CO2 fluxes into the atmosphere, including temperature, moisture and vegetation. Temperature is the main driver of soil respiration, with microbial decomposition having a high activation energy, so soil temperature must be great enough to result in degradation by soil microbes (Davidson & Janssens, 2006). A study by Rustad and Fernandez (1999), showed that a 5 ˚C warming caused a 25-40% increase in CO2 fluxes compared to control plots. This will contribute to a positive feedback system, as increased soil respiration leads to more CO2 in the atmosphere. Soil moisture can also impact soil respiration, with various models suggesting respiration increases from low rates in dry conditions to a maximum at an intermediate moisture level, despite uncertainty in the size of the optimum level (Figure 3). Optimum levels occur in wet soils, but not fully saturated ones as the subsequent anaerobic conditions suppress decomposition (Luo & Zhou, 2006). More abundant precipitation will impact soil respiration in a complex way, as soil moisture may increase towards the optimum level, leading to increased respiration in some areas. However, there could be a decrease in other areas if full saturation is reached, as anaerobic conditions will limit microbial activity.

Soil affects vegetation through influences on water availability, nutrient cycling and temperature regime. Changes in soil conditions affect species composition which can impact the soil organic carbon and soil properties, with varying biomasses returning to soil (Lal, 2004). With climate change, an average rise in mean global annual temperatures of 1° C is equivalent of an approximate 200km poleward shift of vegetation zones (Ozenda & Borel, 1990). This shift in vegetation type will impact soil respiration globally, with a reduction in Tundra and (Northern) bogs that have very low respiration rates (60 and 94 gC/m2/yr), whilst Tropical moist forests, with a rapid rate of 1260 gC/m2/yr, will become more abundant (Raich & Schlesinger, 1992). Shifting vegetation types increase global soil respiration rates, causing a positive feedback loop. However, this will happen over hundreds of years as vegetation adapts and changes through succession.             

This essay has showcased how the varying processes within the earth’s systems are linked, and the various spatial and temporal scales they operate over. The climate-hydrology-soil systems are important not only for determining the various landscapes and vegetation types that occupy climatic regions (vital for human survival through crop cultivation and oxygen turnover), but also through climatic feedback loops. The various positive and negative feedback loops involved in the processes demonstrate how sensitive the earth is to climate change, and how understanding the processes is vital in dealing with climate change and the impacts it will have in the future on earth’s systems and human survival.

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