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

Abstract

Several of the methods used to estimate actual evapotranspiration (ET a ) consider the crop evapotranspiration (ET c ) to be affected by a soil water stress co-efficient (K s ) such that ET a = K s ET c . The aim of this study was to assess the ET a 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 K s . The K s was expressed as either a linear (K s,lin ) or a logarithmic (K s,log ) function of the soil water depletion. Additionally, the method proposed in the FAO-56 (K s,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 ET a were compared with the ET a determined from soil water balance. The ET a estimated with K s,lin had the highest precision (r 2 = 0.67) and accuracy (d = 0.89 and root mean square error (RMSE) = 0.67 mm d −1 ). However, the ET a determined from

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“…e results of the current experiments indicate that this simplification does not deteriorate the final accuracy of the ET A estimates when compared to different approaches. is is in accordance with what has been found by Lyra et al [8], who compared different methods to calculate the soil water stress coefficient.…”

“…e classical Kc approach is widely utilized for operationally determining the water demand of irrigated crops [8,30]. e advantages and limitations of this approach have been investigated in numerous papers and mostly depend on the use of proper Kc values, which can be derived from the existing literature and/or from more modern information sources [31].…”

“…e SWC corresponding to full and null evapotranspiration is assumed to coincide with SWC fc and SWC wp , respectively. is is also a widely used assumption which was found to lead to satisfactory results by Lyra et al [8]. Several authors, however, state that the SWC corresponding to null ET A should actually be lower than field capacity [5,9].…”

“…is assumption is theoretically justified by the mentioned complexity and variability of the actual relationships between ET A and RSWC [13] and is practically necessary to minimize the number of parameters to be identified. As is schematized in Figure 1, in fact, a linear equation relating relative ET A (RET A ) to SWC can be defined identifying the two extreme points, i.e., the SWC corresponding to full and null evapotranspiration, which can be assumed to correspond to 2 Advances in Meteorology field capacity (SWC fc ) and permanent wilting point (SWC wp ), respectively [8]. e identification of SWC wp and SWC fc allows the computation of the soil water stress coefficient on day i, Ks i , from the respective SWC, SWC i , through the following equation:…”

“…Moreover, the collected SWC datasets are often affected by troubles over medium-long time periods due to maintenance problems and to the high variability of measurement conditions which affects soil sensors [6]. Consequently, SWC measurements are indicative of relative SWC variations in time but can hardly be utilized for the quantitative estimation of the soil water balance and ET A [8].…”