Measurement of evapotranspiration and its components in a corn ( Zea Mays L.) field

Abstract

A field experiment was conducted at Prosser, WA in 1993, to evaluate the consistency of field techniques for measuring evapotranspiration (ET), transpiration (T), and soil evaporation (E) from a full-canopy corn field, and to evaluate the ability of sap flow gauges to detect differences in water use of corn under two contrasting irrigation treatments: full irrigation (wet treatment) and irrigation stopped when full canopy was reached (dry treatment). Twenty-minute ET rates for the wet treatment were determined using the Bowen ratio energy balance (BREB) method. Transpiration (sap flow) rates under both irrigation treatments were measured at 20-min intervals using gauges placed on individual corn plants. Daily E rates for the wet treatment were measured using microlysimeters, and also estimated from the difference between BREB, ET and sap flow measurements. Soil–water content in both treatment areas was monitored using neutron probe and gravimetric samples. Considering the normal variation of field measurements, the agreement and consistency among the different techniques used to determine ET and its components was adequate. Despite reductions in leaf area, dry matter production and grain yield, it was possible to differentiate transpiration rates (sap flow) between dry and wet irrigation treatments only at the end of the season. Microlysimeter measurements of daytime E were generally below 1 mm, with an average of 0.6 mm, representing 13.6% of daytime ET. Night time E values were lower, with an average of 0.2 mm. Large coefficients of variation were observed. The use of microlysimeters under low soil evaporation conditions may not be practical, requiring a large number of replications to estimate the average with reasonable confidence. Results also indicated, low agreement between E estimates obtained from the difference between BREB, ET and sap flow and E measured with microlysimeters. The variability of the measurements of sap flow per unit area was large for 20-min intervals, but decreased rapidly for daily to weekly intervals. Sap flow gauges seem capable of measuring weekly flow rates of field corn within ±10% and 80–95% confidence, and daily flow rates with 80% confidence. For shorter time intervals or lower tolerable errors, the number of gauges required increases dramatically.