Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2002
Publication Date: 7/20/2003
Citation: Tyner, J.S., Brown, G.O., Garbrecht, J.D. Incorporation of Chloride Mass Balance to Improve Estimates of Evapotranspiration, Recharge and Runoff. Transactions of American Society of Agricultural Engineers. 2003. v. 46(1). p. 95-103. Interpretive Summary: The potential for groundwater contamination by agricultural fertilizer has been a concern for many years. Assessment of this contamination potential requires that the rate of water flow from the root zone towards the groundwater (recharge) be known. This study presents a method by which the groundwater recharge beneath the root zone can be estimated. The study was conducted on experimental dryland winter wheat plots in Northern Oklahoma near Stillwater. The recharge rate was measured indirectly by use of a conservative chloride tracer that was applied as potassium chloride fertilizer. The recharge estimates from the indirect measurements were further refined by use of a computer model that simulated the movement of water in the soil. The proposed method improved on the traditional method of chloride accounting by providing more detailed information regarding the variability of recharge over time. It also provided more accurate estimates of evapotranspiration and runoff that were consistent with the chloride tracer and soil moisture measurements. The proposed method provides a good alternative to field measurements of evapotranspiration, recharge, and runoff that are resource intensive and impractical to collect over long time periods. In contrast, this method relies on readily available meteorological data, a one-time soil core collection/evaluation, and a computer simulation of soil water flow.
Technical Abstract: A method was presented whereby measured soil water chloride concentrations and long-term precipitation and air temperature were interpreted to estimate evapotranspiration, recharge, and runoff. By applying the chloride mass balance technique, soil water chloride profiles provided a basis for improving the estimated boundary conditions associated with the long-term mean recharge rate. Simple estimates of evapotranspiration and runoff were calculated from precipitation and air temperature data. Then measured precipitation, and estimated evapotranspiration and runoff were used to drive an unsaturated groundwater model to estimate recharge. This estimated recharge was compared to the long-term mean recharge rate calculated from the chloride profiles. Finally, the evapotranspiration and runoff components of the model were scaled such that the modeled recharge rate was similar to the long-term mean recharge rate. This method improved the classical application of chloride mass balance, which only provided a mean recharge, but not a time distribution of recharge. In addition, the method can be used to scale the estimated evapotranspiration and runoff so that the resulting estimates of evapotranspiration and runoff were consistent with chloride and water mass balance equations, thereby increasing their accuracy. Although measurements of evapotranspiration, recharge, and runoff are more direct, they are resource intensive to collect over long time periods. In contrast, this method relies on basic meteorological data and soil cores collected at a single point in time.