Comparison of calculated and measured volumetric water content at four field sites

Terrestrial field dissipation studies are currently required as part of new active ingredient submission packages in many countries. Results from field dissipation studies are used to determine the fate of pesticides under field conditions which includes evaluation of compound degradation, mobility and other routes of dissipation. Time domain reflectometry (TDR) instrumentation has been helpful for determining if water movement through the soil profile was correlated with or was sufficient to cause compound mobility. A simple method is presented for estimating soil water flux from the soil profile using TDR technology. Additionally the question has been raised regarding how useful simple calculations of volumetric water content (VWC) can be for this purpose as well. A simple method of calculating VWC and profile flux are presented and compared to measurements using TDR and TDR based flux calculations. Results from this comparison indicate that calculated VWC estimates compare favorably with those based on actual TDR measurements. Calculated patterns of flux at a 120 cm depth based on TDR and the calculated VWC method correlate well, although the flux quantities can vary. Occurrence of flux from both methods were very similar, but quantities of flux were greater with the calculated VWC method compared to flux quantities based on the TDR calculation method. Additionally, because the calculated VWC method is computationally simple, the technique should also be useful for estimation of groundwater recharge at a regional scale using tools such as GIS.