Modeling water balance models

Data on weather conditions, especially temperature and rainfall (temporal distribution and intensity) in the study area are essential for the evaluation of the dissipation data. It is very important to understand the water balance in the paddy field as accurately as possible when calculating the rate of outflow. Records of changes in water temperature and sediment temperature are also helpful for modeling the behavior of a chemical in the rice paddy field. 

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. 

Surface runoff (Q) is the second-largest part of the hydrologic water balance for ET landfill covers at many sites in humid regions. Even at dry sites where surface runoff is small, errors in estimates of Q are important, and especially so if the model estimates significant Q on days with no runoff. Estimates of Q are therefore important to the design process at all sites. 

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. 

The EPIC model is a comprehensive model that has been extensively tested for water balance estimates in dry and wet climates, including sites with significant accumulation of snow in winter.14... 

The primary purpose of the HELP model is to provide water balance data with which to compare design alternatives for conventional barrier-type covers installed on landfills with bottom liners. It provides a tool for both designers and permit writers and is applicable to open, partially closed, or fully closed sites. 

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. 

Both of them require at least limited model calibration. They do not stochastically estimate daily climate data for model evaluations or long-term changes in plant nutrient status and the resulting changes in plant growth and water balance. HYDRUS and UNSAT-H would be very useful and accurate if used in research however, they are difficult to use in engineering design of ET landfill covers and provide incomplete estimates of performance. 

Khire, M.V., Benson, C.H., and Bosscher, P.J., Water balance modeling of earthen final covers, Journal of Geotechnical and Geoenvironmental Engineering, 123, 744-754, 1997. 

The SWCR system is built on top of the FMC. The purpose of the SWCR system is to prevent infiltration of surface water into the landfill by containing and systematically removing any liquid that collects within it. Actual design levels of surface water infiltration into the drainage layer can be calculated using the water balance equation or the HELP model.36 37... 

SWRRB - The Simulator for Water Resources on Rural Basins (SWRRB) was developed at EPA by R. Carsel and is a modification of the USDA model CREAMS (. It was orginally developed to predict daily runoff volume for small watersheds throughout the U.S. The basic runoff model is based on the water balance equation ... 

The physical mechanism of membrane water balance and the formal structure of modeling approaches are straightforward. Under stationary operation, the inevitable electro-osmotic flux has to be compensated by a back flux of water from cathode to anode, driven by gradients in concentration, activity, or liquid pressure of water. The water distribution in PEMs that is generated in response to these driving forces decreases from cathode to anode. With increasing/o, the water distribution becomes more nonuniform. the water content near the anode falls below the percolation threshold of proton conduction, X < X. This leaves only a small conductivity due to surface transport of water. As a consequence, increases dramatically this can lead to failure of the complete cell. 

The challenge for modeling the water balance in CCL is to link the composite, porous morphology properly with liquid water accumulation, transport phenomena, electrochemical kinetics, and performance. At the materials level, this task requires relations between composihon, porous structure, liquid water accumulation, and effective properhes. Relevant properties include proton conductivity, gas diffusivihes, liquid permeability, electrochemical source term, and vaporizahon source term. Discussions of functional relationships between effective properties and structure can be found in fhe liferafure. Because fhe liquid wafer saturation, 5,(2)/ is a spatially varying function at/o > 0, these effective properties also vary spatially in an operating cell, warranting a self-consistent solution for effective properties and performance. 

Recently. Weber and Newman " " introduced a framework for bridging the gap between the Bernard and Verbrugge and the Springer et al. membrane approaches. The membrane model was used in a simple fuel-cell model, and it showed good agreement with experimentally measured water-balance data under a variety of conditions. The fuel-cell model was similar to the model of Janssen. who used chemical potential as a driving force in the... 

The simplest way to treat the catalyst layers is to assume that they exist only at the interface of the diffusion media with the membrane. Thus, they are infinitely thin, and their structure can be ignored. This approach is used in complete fuel-cell models where the emphasis of the model is not on the catalyst-layer effects but on perhaps the membrane, the water balance, or multidimensional effects. There are different ways to treat the catalyst layer as an interface. 

In addition, the models demonstrated that the water balance is important and can alter potential and reactant location. For example, the reaction distribution is closer to the channel than the rib near the inlet regions for humidified feeds. The reason is that, as the water moves from anode to cathode, there is more flooding, but the movement of water in this direction decreases along the channel because of lower current densities associated with lower oxygen... 

Zappa M, Pos F, Strasser U (2003) Seasonal water balance of an alpine catchment as evaluated by different methods for spatially distributed snowmelt modelling. Nordic Hydrol 34(3) 179-202... 

Merz J (2003) Water balances, floods and sediment transport in the Hindu Kush-Himalayas. Data analysis, modeling and comparison of selected meso-scale catchments. Geographica, Bemensia G72 339... 

Gerten, D. (2004). Terrestrial vegetation and water balance—Hydrological evaluation of a dynamic global vegetation model. J. Hydrol. 286(1 4), 249-270. 

Numerical model of global water balance 4.6.4.1 Modeling the global water cycle... 

Figure 4.14. The block scheme of the sample model of water balance in a small territory.

The diversity of forms to parameterize the dependence of the rate of evaporation from the soil surface on environmental parameters facilitates adapting the model of water balance to the information base. 

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