No abstract

With a view toward developing techniques for studying water flow in spatially varying soils, field data for soil water characteristic relationships and unsaturated hydraulic conductivity are scaled by using the concept of similar media. Data observed by different investigators at three geographic areas are used. The soil water characteristic data consisted of 840, 900, and 512 observations, while those for the unsaturated hydraulic conductivity (available from only one of the three sources) consisted of 2640 observations. In the process of scaling the data a best fit for the scaled data is defined in terms of a sum of squares about an ‘average’ curve using one value for the scaling parameter for each sampling location. Comparisons made between curves fitting the data and those fitting the scaled data show that scaling reduces the sums of squares by amounts varying from 34 to over 90%. For similar media the scaling parameter determined at a given sampling location for the soil water characteristic relationship (α from h(S)) should be identical to that for the unsaturated hydraulic conductivity (αr from K(S)). Although these parameters were highly correlated (r = 0.91 ) for the only set of data available, α r from h(S) values are shown to be more effective in scaling the unsaturated hydraulic conductivity data than are αr from K(S) values in scaling soil water characteristic data.

Abstract. The multiyear, root zone soil moisture redistribution characteristics in a semiarid rangeland in southeastern Arizona were evaluated to determine the magnitude and variability of deep-profile, wintertime soil moisture recharge. Intermittent observations from 1990 to 1998 of average volumetric soil moisture under shrub and grass cover showed that significant recharge beyond 0.30 rn principally occurs only in the wintertime when the vegetation is senescent and does not use the infiltrating water. Using the physically based, variably saturated flow model HYDRUS, wintertime observations were modeled to determine the recharge of soil moisture at different depth intervals in the vadose zone. Two approaches were carried out to estimate the soil model parameters. The first was to use basic soils data from detailed profile descriptions in conjunction with pedotransfer functions. The second parameter estimation strategy was to use an automatic parameter search algorithm to find the optimal soil parameters that minimize the error between the model-computed volumetric water content and observations. Automatic calibration of the model was performed using the shuffled complex evolution algorithm (SCE-UA), and it proved possible to satisfactorily describe the vadose zone observations using a simplified description of the soil profile with optimal model parameters. Simulations with the optimized model indicate that significant recharge of vadose zone does occur well beyond 0.30 m in winter but that such recharge is highly variable from year to year and appears correlated with • E1 Nifio episodes. This water could serve as a source of plant water for deeper-rooted plants that are active during the subsequent spring season, thereby exploiting a niche that the more abundant, shallower-rooted plants that are active during the summer rainy season do not. However, the year-to-year variability of the winter precipitation and consequent deep soil moisture recharge indicates that the deeper-rooted vegetation in this region must retain the ability to obtain moisture from the near surface in order to meet its water demands if necessary. IntroductionIn this paper, we document the root zone soil moisture redistribution processes that occurred during an 8 year time period at two rangeland sites in the semiarid southwestern United States. Our approach was to use a variably saturated hydrological flow model to represent intermittent soil moisture profile observations and in this way to determine the wintertime soil moisture recharge rates. To model the observations accurately, it is necessary to derive effective parameters for the model. An additional facet of this work therefore is to demonstrate the feasibility of using an optimization methodology and to compare this parameter estimation approach with a traditional one that uses basic soils data in conjunction with pedotransfer functions. On the basis of our modeling study we examine the hydrologic feasibility of the proposition that wintertime soil moisture recharge in southeaster...

Water flow from a point source is analyzed using a linearized form of the moisture flow equation. Time‐dependence is assumed with the results simplifying to those of previous investigators for steady‐state conditions. Discrete time‐distributed inputs such as might occur for trickle or high frequency irrigation is amenable to the solution. Numerical simulations include (i) the advance of a wetting front during infiltration, (ii) moisture variation resulting from a cyclic input as during irrigation, and (iii) the matric flux potential field for a two‐source problem.

No abstract