Soil water stress co-efficient for estimating actual evapotranspiration of maize in northeastern Brazil

Abstract

Several of the methods used to estimate actual evapotranspiration (ETa) consider the crop evapotranspiration (ETc) to be affected by a soil water stress co-efficient (Ks) such that ETa = Ks ETc. The aim of this study was to assess the ETa estimation in maize in Alagoas State (09 ° 38 ′ S and 36 ° 40 ′ W; 260 m msl) in northeastern Brazil using the method of the single crop co-efficient from the FAO-56 paper, combined with three different approaches to determine Ks. The Ks was expressed as either a linear (Ks,lin) or a logarithmic (Ks,log) function of the soil water depletion. Additionally, the method proposed in the FAO-56 (Ks,FAO) was used when the impact was linear, but it was switched on after only one offset. Meteorological and soil water measurements were conducted in a maize crop from July to October 2005. The estimates of ETa were compared with the ETa determined from soil water balance. The ETa estimated with Ks,lin had the highest precision (r2 = 0.67) and accuracy (d = 0.89 and root mean square error (RMSE) = 0.67 mm d−1). However, the ETa determined from Ks,log (r2 = 0.41, d = 0.57 and RMSE = 1.69 mm d−1) or Ks,FAO (r2 = 0.46, d = 0.62 and RMSE = 1.73 mm d−1) was not estimated successfully. When the parameters of Ks,FAO were fitted to the local edaphoclimatic conditions, the precision (0.67) and accuracy (0.89 and 0.82 mm d−1) were similar to the estimates with Ks,lin but with a better performance for ETa > 3.0 mm d−1.