Description
The soil moisture balance method for estimating groundwater recharge assumes that a minimum amount of infiltration is required before recharge may occur. This minimum condition is typically assumed to be field capacity. The method a1so assumes that after this minimum is reached, all infiltration recharges the saturated zone. Consistent discrepancies between field and laboratory specific retention tests indicate the need to reevaluate the above assumptions. The objective of this study is to evaluate the discrepancies between laboratory and field specific retention tests and to determine if the concepts of specific retention and field capacity are relevant to estimations of groundwater recharge. The objective is addressed by answering the following six questions: 1) Is there a minimum amount of infiltration required before groundwater recharge occurs? 2) If so, how does the minimum infiltration value relate to field capacity? 3) Does all infiltration that occurs after field capacity has been satisfied recharge the groundwater? 4) Do field capacity measurements provide moisture contents that are at steady state equilibrium with the water table? 5) Can the moisture contents from the field capacity measurements be re1ated to the characteristic curve? 6) Why is there a consistent discrepancy between the results of the laboratory and field procedures used by consultants to measure field capacity? Field work was conducted at one location in the Fallbrook series, B phase soil in East San Diego County. Four different methods were used to measure specific retention: (1) laboratory drainage, (2) small-area field drainage, (3) large area field drainage, and (4) characteristic curve inflection point. The effective depth of evaporation was estimated on the basis of field hydraulic head gradient measurements and numerical model results. A one-dimensional unsaturated flow model was written to determine the minimum amount of water required to infiltrate a dry soil profile prior to recharge (critical infiltration value). The model was also used to determine the moisture content distribution of a draining soil over an extended period of time. Additionally, numerical simulations were also conducted to determine the importance of the lower boundary condition on 1aboratory specific retention tests. Severa1 methods were used to create characteristic and conductivity curves to use as a data base for the model. Field measured characteristic curves and pressure plate derived conductivity curves were the input data actually used by the model. Based on the results from the specific retention procedures and the numerical model the following conclusions can be made. A minimum amount of infiltration is required prior to groundwater recharge. This minimum is equivalent to the available water capacity as measured by the large-area field drainage procedure. All infiltration in excess of the available water capacity does not recharge the groundwater. The amount lost to evapotranspiration is significant. Specific retention values measured in this study were not in equilibrium with the water table elevation nor are they related to the soil's characteristic curves. The small-area field specific retention test is vertical subject to high horizontal tension gradients that unrealistically limit the extent of vertical flow. Laboratory specific retention tests are controlled by the proximity of the atmospheric lower boundary and the PET rate in the 1aboratory. Results of these methods are representative of field conditions. The Soil Moisture Balance method may be able to provide reasonable estimates of groundwater recharge if the amount of infiltration lost to evapotranspiration, after the available water capacity is met, could be estimated. The increased cost of such an analysis, in conjunction with the increased cost to properly measure available water capacity, advocates that a numerical model to estimate recharge be developed.
