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One-step approach for estimating maize actual water use: part II. Lysimeter evaluation of variable surface resistance models

López-Urrea, R., Chávez, J. L.
Irrigation science 2019 v.37 no.2 pp. 139-150
Zea mays, corn, equations, evapotranspiration, humid zones, irrigation, lysimeters, models, plant cultural practices, semiarid zones, solar radiation, Texas
This study evaluated maize variable bulk surface resistance (rₛ, s m⁻¹) models developed using field/environmental data from non-irrigated fields in a humid climate. The different rₛ models were reported in the companion paper. Surface resistance values derived from the application of the new models were inserted in the 1965 Penman–Monteith ET equation to calculate actual maize evapotranspiration (ETₐ). This is the so-called one-step approach. The evaluation was performed with maize water use data measured with a large weighing lysimeter located in the middle of an irrigated maize field (semi-arid climate) near Bushland, Texas, USA. The evaluation was performed for different time scales (semi-hourly to daily) and the rₛ models bias (MBE) and root mean square errors (RMSE) were determined. Using daytime 30-min rₛ models in maize ETₐ (mm 30-min⁻¹) estimation resulted with the lowest relative error of 15.4%. While daytime average rₛ models developed from daytime average explanatory variables resulted with the lowest relative error of 20.9% in 30-min maize ETₐ estimation. When rₛ models from 30-min and daylight average explanatory variables were used to obtain cumulative daytime maize ETₐ estimations, resulting errors were lower than for semi-hourly time step. In addition, when daytime 30-min and average rₛ models were applied in conjunction with a fixed rₛ nighttime value to obtain daily maize ETₐ, results were similar than those for the daytime time step. Furthermore, another evaluation incorporated rₛ values obtained adopted ranges of crop evaporative fraction (EF). When this EF-based rₛ values were used to estimate maize (ETₐ, mm 30-min⁻¹) the error found was 5.9 ± 21.7%. This result seems high; however, when the EF range-based rₛ values were applied in the estimation of maize cumulative daytime ETₐ, results indicated an error of 5.9 ± 11.9%. In this last case, the relative error was much lower than for 30-min ETₐ estimations. Therefore, it is concluded that some maize rₛ models, reported in the companion paper, performed well when evaluated in a different climate and agronomic practices and are suitable for the estimation of ETₐ using the 1965 Penman–Monteith ET equation. In particular daytime 30-min rₛ models applied to 30-min intervals of ETₐ estimation and then accumulated over the day performed better than the others rₛ models and time steps studied.