SAMPLING STRATEGIES FOR SOIL-WATER CONTENT TO ESTIMATE EVAPOTRANSPIRATION

When the soil water balance method is applied at a field scale, estima tion of the spatial variability and confidence interval of actual evap otranspiration is rare, although this method is sensitive to the spati al variability of the soil, and thus to the sampling strategy. This wo rk evaluated the effect of soil sampling strategies for soil water con tent and water flux at the bottom of the soil profile on the estimatio n of the daily and cumulative evapotranspirations. To do that, accordi ng to the statistical properties of daily measurements in a field expe riment with a soybean crop, the water content and flux through the bas e to the soil profile in space (field scale) and time (daily scale) we re simulated. Four different sampling strategies were then compared, a nd their effects on daily and seasonal cumulative evapotranspirations quantified. Strategy 1 used ten theoretical sites randomly located in the field. The daily water content estimates were assumed to be availa ble each day from these same ten locations, which were located from 0. 15 m to 1.55 m in depth, with space steps of 0.10 m. Strategy 2 assume d that daily water content estimates combined two sources: in the 0.00 -0.20 m soil layer, ten theoretical sites were selected but changed ev ery day, with thin soil layers for soil moisture sampling, from 1 to 5 cm in thickness. In the 0.20-1.60 m soil layer, the daily water conte nt estimates were assumed to come from the same ten locations (the fir st soil moisture estimate was located at 0.25 m, and the others were l ocated every 0.10 m until 1.55 m). Strategy 3 used ten theoretical sit es located in the field, as in strategy 1, however the water content e stimates in the 0.00-0.20-m soil layer were assumed to come from accur ate water content measurements (soil layers from 1 to 5 cm in thicknes s), while for the 0.20-1.60 m soil layer, the strategy was similar to strategies 1 and 2. Strategy 4 used 10 new theoretical locations of me asurement every day. Precise water content estimates for thin layers w ere assumed to be available in the 0.00-0.20 m soil layer as in strate gy 2. The layers for water content estimates in the 0.20-1.60 m were s imilar to those of strategies 1, 2, and 3. Results showed that the spa tial variability of the daily actual evapotranspiration may not be neg ligible, and differences from approximately +/- 1.0 mm d-1 to +/- 3.0 mm d-1 were calculated between the four sampling strategies. Strategy 1 gave the worst results, because variations in the water content of t he top soil layers were neglected, and thus the daily evapotranspirati on was underestimated. Strategy 2 led to a considerable variability fo r estimating daily evapotranspiration which was explained by the effec t of the spatial variability due to the daily site sampling for the to p soil layers (0 to 0.2 m). Strategy 3 appeared to be the best practic al compromise between practical field considerations and the necessity to obtain accurate evapotranspiration measurements. The accuracy of d aily evapotranspiration could reach +/- 0.5 mm d-1, and could be furth er improved by increasing the number of measurement sites. The best re sults were obtained with strategy 4, although such a destructive and t ime-consuming strategy is not likely to be practical.