Soil water balance

Gardner, L. R., and Reeves, H. (2002). Seasonal patterns in the soil water balance of a Spartina marsh site at North Inlet, South Carolina, USA. Wetlands 22, 467—477. 

The presence of water is critically important to most of the physical, chemical and biological processes that occur within soils and is expressed as a soil water balance . Water balance expresses the difference between water use and water need, i.e. the water stored in soil that affects soil (micro)biology and plant growth (Section 4.6.4). 

The effect of all the various phenomena connected with the water circulation between the atmosphere, biosphere and hydrosphere is reflected in general in the water regime of the soil. The water regime includes the water penetration into the soil, its motion and retention in the soil, and its escape from the soil. It is characterized quantitatively by the soil water balance, which may be expressed by the relationship ... 

Water balance calculations aided by geology and soil data... 

Figure 7 shows schematically the main components of the water balance in soils, the integral equation for which in terms used in the figure can be written as follows ... 

Fig. 7. Main contributions to the water balance in soil-plant system...

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Table 6 Soil characterization results used in water balance calculations and data interpretations...

Kochi a residue levels of 2.5% significantly altered sorghum water balance, and plants grown in soil with cocklebur residues above the growth-inhibition threshold showed a trend toward elevated leaf resistances and lower water potentials than controls. A lower growth-... 

Alternative final cover systems, such as the innovative evapotranspiration (ET) cover systems, are increasingly being considered for use at waste disposal sites, including municipal solid waste (MSW) and hazardous waste landfills when equivalent performance to conventional final cover systems can be demonstrated. Unlike conventional cover system designs that use materials with low hydraulic permeability (barrier layers) to minimize the downward migration of water from the cover to the waste (percolation), ET cover systems use water balance components to minimize percolation. These cover systems rely on the properties of soil to store water until it is either transpired through vegetation or evaporated from the soil surface. 

In addition to being called ET cover systems, these types of covers have also been referred to in the literature as water balance covers, alternative earthen final covers, vegetative landfill covers, soil-plant covers, and store-and-release covers. 

Numerical models are used to predict the performance and assist in the design of final cover systems. The availability of models used to conduct water balance analyses of ET cover systems is currently limited, and the results can be inconsistent. For example, models such as Hydrologic Evaluation of Landfill Performance (HELP) and Unsaturated Soil Water and Heat Flow (UNSAT-H) do not address all of the factors related to ET cover system performance. These models, for instance, do not consider percolation through preferential pathways may underestimate or overestimate percolation and have different levels of detail regarding weather, soil, and vegetation. In addition, HELP does not account for physical processes, such as matric potential, that generally govern unsaturated flow in ET covers.39 42 47... 

The ET cover cannot be tested at every landfill site so it is necessary to extrapolate the results from sites of known performance to specific landfill sites. The factors that affect the hydrologic design of ET covers encompass several scientific disciplines and there are numerous interactions between factors. As a consequence, a comprehensive computer model is needed to evaluate the ET cover for a site.48 The model should effectively incorporate soil, plant, and climate variables, and include their interactions and the resultant effect on hydrology and water balance. An important function of the model is to simulate the variability of performance in response to climate variability and to evaluate cover response to extreme events. Because the expected life of the cover is decades, possibly centuries, the model should be capable of estimating long-term performance. In addition to a complete water balance, the model should be capable of estimating long-term plant biomass production, need for fertilizer, wind and water erosion, and possible loss of primary plant nutrients from the ecosystem. 

Because borrow soils will be mixed and modified during placement, the cover soil for an ET landfill cover, as constructed, will be unique to the site. However, the soil properties may be easily described. The design process requires an evaluation of whether or not the proposed soil and plant system can achieve the goals for the cover. Numerous factors interact to influence ET cover performance. A mathematical model is needed for design that is capable of (1) evaluating the site water balance that is based on the interaction of soil, plant, and climate factors and (2) estimating the performance of an ET landfill cover during extended future time periods. 

ET is the evaporation of water from the soil surface and by plant transpiration (primarily through the stomata on the plant s leaves). ET should be carefully considered during all stages of design since it will be the largest mechanism of water removal in the water balance for an ET cover. With current knowledge, it is necessary to estimate potential evapotranspiration (PET) first and then using the PET estimate the actual evapotranspiration (AET) for the site. 

Soil provides the medium in which plants grow it stores precipitation within the ET cover and provides nutrients for plant growth. Total (potential) soil-water storage capacity is controlled by soil properties. The storage capacity available at any instant in time is controlled primarily by the balance between infiltration from precipitation and rate of water removal from the soil by ET. 

If properly designed, the soil-water reservoir of an ET cover will be only partially filled most of the time. The greatest amount of water that must be stored in the soil will be defined by major or critical events. The critical event may result from a single storm event or a series of storms. The model used for design or evaluation of an ET landfill cover should be capable of evaluating the cumulative effect of each day s water balance activity and thus identify critical events. 

The density of soil may control the presence, absence, or density of roots found in a particular soil layer. The density of plant roots in a soil layer determines how much water plants can remove from the layer and its rate of removal. Soil compaction, in addition to inhibiting root growth, reduces soil-water-holding capacity. A model that does not consider the effect of soil density on water balance may produce significant errors in water balance estimates. 

Development of the Environmental Policy Integrated Climate (EPIC) model and its predecessor, the Erosion Productivity Impact Calculator, began in the early 1980s.69 70 The first version of EPIC was intended to evaluate the effects of wind and water erosion on plant growth and food production. More recent versions also evaluate factors important to other environmental issues. EPIC is a onedimensional model however, it can estimate lateral flow in soil layers at depth. All versions of EPIC estimate surface runoff, PET, AET, soil-water storage, and PRK below the root zone—these complete the hydrologic water balance for an ET landfill cover. 

In addition to a complete water balance, EPIC estimates plant biomass production, fertilizer use, wind and water erosion, loss of nitrogen and phosphorus from the soil, and the effect of nutrient loss from the soil on plant growth. 

UNSAT-H does not address the effects of soil density on plant growth and water balance. Disadvantages caused by the computational methods used to estimate soil water flow include the following (1) the model requires the user to choose from several submodels to solve the Richards equation this choice should be made by a person with training in advanced soil physics and (2) the model requires the input of several soil parameters that are difficult to estimate for the completed cover soil. 

Fayer, M.J. and Gee, G.W., Multiple-year water balance of soil covers in a semiarid setting, Journal of Environmental Quality, 35 (1), 366-377, 2006. 

Without the processes of settling and decanting (and wasting sludge), this is precisely the same process as treating a contaminant in a soil site. Allowances have to be made for water balance, but the process is the same. 

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