Maize Evapotranspiration Estimation Using Penman-Monteith Equation and Modeling the Bulk Canopy Resistance

Nora Meraz-Maldonado,Héctor Flores-Magdaleno

doi:10.3390/w11122650

Abstract

Some techniques, such as the Katerji and Perrier approach, estimate the bulk canopy resistance (rc) as a function of meteorological variables and then calculate the hourly evapotranspiration using the Penman–Monteith equation, so that traditional crop coefficients are not needed. As far as we know, there are no published studies regarding using this method for a maize crop. The objective of this study was to calibrate and validate the canopy resistance for an irrigated continuous maize crop in the Midwestern United States (US). In addition, we determined the effect of derivation year, bowen ratio, and the extent of canopy. In this study we derive empirical coefficients necessary to estimate rc for maize, five years (2001–2005) were considered. A split-sample approach was taken, in which each year’s data was taken as a potential calibration data set and validation was accomplished while using the other four years of data. We grouped the data by green leaf area index (GLAI) and the Bowen ratio (β) by parsing the data into a 3 × 3 grouping: LAI (≥2, ≥3, and ≥4) and |β| (≤0.1, ≤0.2, and ≤0.3). The best fit data indicated reasonably good results for all nine groupings, so that the calibration coefficients derived for the conditions LAI ≥ 2 and |β| ≤ 0.3 were taken in light of the longer span associated with LAI ≥ 2 and the larger number of hours. For the calibrations in this subgroup, the results indicate that the annual empirical coefficients for rc are nearly the same and equally effective, regardless of the year used for calibration. Our validation included all the daytime hours regardless of β. Thus, it was concluded that the calibration at our site was independent of the derivation year. Knowledge of the Bowen ratio was useful in calibration, but accurate ET estimates (validation) can be obtained without knowledge of the Bowen ratio. Validation resulted in hourly ET estimates for irrigated maize that explained 83% to 86% of the variation in measured ET with an accuracy of ± 0.2 mm.

Highlights

Irrigation is the largest use of water resources in many agricultural regions of the world.The challenges of planning for water use and water resources management in a changing climate include the increased stress on the world’s fresh water reserves due to increased use of water for food production, contamination of rivers, lakes, and aquifers, and a lack of support for major new water projects
The solution is for the conditions green leaf area index (GLAI) ≥ 2 and|β| ≤ 0.2, which correspond to the top middle part of Table 1
Contributing to the inconsistency from year to year is the fewer number of hours in the more restrictive conditions of |β| ≤ 0.1, which result in a smaller range in r*/ra

Summary

Introduction

The challenges of planning for water use and water resources management in a changing climate include the increased stress on the world’s fresh water reserves due to increased use of water for food production, contamination of rivers, lakes, and aquifers, and a lack of support for major new water projects. There is potential need to alleviate the stress on the world’s fresh water system with human population increasing and the need for more food production; conserving water through irrigation scheduling provides that potential [1]. The method, frequency, and duration of irrigation strongly affect crop yields and farm productivity [2]. An efficient use of water resources through irrigation scheduling requires an accurate estimation of crop water use to realize a potential savings.

Objectives

Methods

Results

Discussion

Conclusion