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New Approaches in Water Balance Computations (Proceedings of the Hamburg Workshop, August 1983). IAHS Publ. no. 148.
Soil water hydrology: simulation for water balance computations
KEITH E. SAXTON USDA-ARS, Agricultural Engineering Department, Washington State University, Pullman, Washington 99164, USA
ABSTRACT Soil moisture occurs as the result of the climatic, crop, and soil processes which impact water on a vegetated landscape. This complex and highly dynamic water supply provides the water upon which the vast majority of the world food supply depends. Most importantly, it is as variable within and between years and location as the variables which determine its occurrence. The physical processes which determine the dynamic state of soil water have been mathematically described and combined into several simulation models in recent years for agricultural and hydrological objectives. These vary in complexity, inputs, and products according to their objective and developers. Several of these models have been applied to the simulation of daily estimates throughout the crop year. The SPAW model developed by the author has been applied to a variety of situations and objectives. This model will be briefly described followed by a discussion of several recent trial applications. These results and those of other similar models suggest that we now can use simulation models to integrate existing knowledge with current observation to provide dynamic soil water estimates useful for several important agricultural and hydrological assessment and prediction needs.
Hydrologie de 1'humidité du sol: simulation pour les calculs du bilan hydrologique RESUME La teneur en humidité du sol résulte des processus climatiques, du type de culture et des processus du sol qui ont une influence sur un paysage couvert de végétation. Cette humidité du sol, à la fois complexe et dynamique, est à la base des apports en eau dont dépend la majorité des approvisionnements en vivres de la planète. D'une importance prépondérante est le fait que sa variabilité entre les années et à l'intérieur de chaque année et entre les différents sites est aussi grande que pour les variables qui déterminent sa présence. Ces dernières années, les processus physiques qui déterminent l'état dynamique de l'humidité du sol ont été décrits mathématiquement et combinés dans plusieurs modèles de simulation pour des objectifs hydrologiques et agricoles. Il y a de notables différences quant à la complexité, les données d'entrée et les résultats suivant l'objectif et la
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personalité de ceux qui les ont mis au point. Plusieurs de ces modèles ont été utilisés pour la simulation des estimations journalières tout au long d'une culture. Le modèle SPAW, mis au point par l'auteur, a été appliqué dans plusieurs situations et pour des objectifs différents. Le modèle sera brièvement décrit puis on discutera de plusieurs essais récents. Les résultats, ainsi que ceux d'autres modèles similaires, suggèrent que nous pouvons maintenant utiliser les modèles de simulation pour intégrer les connaissances existantes avec les données d'observation afin de réaliser des estimations dynamiques de l'humidité du sol que l'on peut utiliser pour plusieurs besoins importants d'estimation et dans le domaine de l'agriculture et de l'hydrologie.
The occurrence and distribution of soil water is a complex and integral part of any hydrological water balance. Whether the emphasis is on surface runoff, streamflow, évapotranspiration, or groundwater, soil water plays a dominant role. Every water balance scheme or hydrological model makes some attempt to predict the occurrence and impact of soil water. The magnitude and time-depth distribution of water in the upper 1-2 m of the earth's mantle readily identify why soil water is of significant importance. Within the hydrological cycle, the storage and release of soil water is second in magnitude only to the precipitation source itself. In climates with less than 400 mm precipitation, virtually all of the precipitation enters the shallow soil layers to be transferred back to the atmosphere by évapotranspiration (ET). In progressively wetter climates, the soil mantle is more likely to reach a state of high wettness or saturation resulting in more surface runoff and throughflow to deeper layers and groundwater.
The importance of soil water is readily apparent when reviewing a description of almost any hydrologie or agricultural system. The infiltration and évapotranspiration processes, in particular, are strongly related to the time-depth status of the soil-water profile. Most groundwater recharge occurs only after the soil profile becomes significantly wetted. Crop production is highly dependent on having adequate available soil water throughout the growing season. Any attempt to estimate crop yields must include soil-water effects.
The objective of this paper is to briefly review recent advances in the simulation of soil water with particular emphasis on those aspects pertinent to soil-water balance computations for hydrologie and agricultural applications. Mathematical representations of the major processes will be described and simulation models reviewed which integrate these into predictive systems.
Numerous developments have occurred in recent years that have significantly enhanced the knowledge and predictability of soil water occurrence. Most of these have been incorporated into computer
Soil water hydrology 49
simulation methods, which in themselves have provided insight and predictive capability. While too numerous to describe fully, several advancements merit review and referencing.
Soil water occurs as a result of several processes that operate within the time line either sporadically or more continuously. Very simply, precipitation is the principle resupply, soil porosity is the storage medium, and évapotranspiration accounts for the majority of depletion. The variables and process equations involved within this total system are numerous and complex. The ability to assemble these equations into computer simulation methods has been the most important advance in recent years.
The inputs and descriptions for these soil water simulations can be categorized into those related to climate, soils, and plants. Climate includes both precipitation and those variables related to the ET potential.
Daily precipitation is the most common input to soil water balance models. Measured values are most often used if available near the study site. But recent efforts for long-term studies or locations away from measurement locations have caused the development of improved stochastic methods. Richardson (1981) and Woolhiser & Roldan (1982) report good success with a spatially distributed parameter model which maintains the statistical characteristics of the regional precipitation pattern while providing random-based sequences. Orsborn et al. (1982) provide a useful summary to several methods and considerations for precipitation inputs to water budget simulât ions.
Most évapotranspiration methods begin with a climatically deter- mined potential ET rate for each day (Saxton et al., 1974a). The inputs vary from only air temperature or pan evaporation to a full complement of radiation and aerodynamic descriptions. It is often necessary to estimate these inputs; and, again, recent progress has been made. The methods of Richardson (1981) in particular provided good statistical methods based on limited data. Intercorrelations among precipitation, solar radiation, and air temperatures were considered in their techniques.
Vegetation plays a significant role in the ET process; thus to simulate soil water requires an adequate description of the growth state and water-related functions. This is especially important where agricultural crops are involved because they rapidly change over the course of a growing season. Descriptions of this biological phase of water balance computations are some of the most difficult. The above-ground growth is often described by time distributions of crop canopy or leaf-area index (Saxton & McGuiness, 1982) and include crop residues. The phenologic development may be a part of crop coefficients (Jensen, 1973; Wright, 1982) or described separately (Saxton et al., 1974b).
Root effects are perhaps the least understood. They have been described by some as a time-varying rooting depth with a density distribution (Ritchie, 1972), or an effective water uptake time-depth distribution (Saxton et al., 1974b). Taylor & Klepper (1973) provide a detailed description for corn, but much actual detail as to how to mathematically represent the root effectiveness is still lacking.
Plant water stress is another area difficult to represent mathem- atically. Leaf-water potentials and resistances through the leaves,
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stems, and roots have been described and measured (Salisbury & Ross, 1978; Gates, 1980). Some have incorporated this approach even though definitive numbers are yet very empirical. The work of Denmead & Shaw (1962) provides a summary of empirical relationships which incorporate the plant response in relation to atmospheric demand and available soil water.
Methods to describe the soil characteristics for holding and transmitting water have received considerable recent advancement. While the concepts of soil tensions and conductivities in relation to soil features have been described for decades,