Lysimetry versus Neutron Moisture Meter for Evapotranspiration Determination in Four Soils

Knowledge of evapotranspiration (ET) is vital for the management of our freshwater resources. One method for determining ET is through the measurement of the soil water balance, where ET is the residual calculated from the change in soil water storage plus precipitation and irrigation and minus drainage and runoff. The objective of this research was to compare the ET calculations where the change in soil water storage was measured using the neutron moisture meter (NMM), or ETNMM, vs. using weighing lysimeters (ETLYS) in four soils. Cotton (Gossypium hirsutum L.) was grown in 2006 and 2007 in weighing lysimeters with NMM access tubes and drainage systems. The soil textures ranged from fine sand to clay loam. The ETNMM was <= 4% of the ETLYS for the clay loam and sandy loam soils, but ETNMM was 8% less than ETLYS in the fine sand due to errors created by the timing of drainage and NMM measurements. At ETLYS amounts <50 turn, the difference between ETLYS and ETNMM for individual measurement intervals could be as much as 28 mm and the average ETNMM/ETLYS ratios as much as 1.20. Beyond 100 turn of ETLYS, the average ETNMM/ETLYS ratios became near 1.0 except for the fine sand, where unmeasured drainage out of the NMM sensor zone resulted in an underestimation of ET. When all other soil water balance components were quantified, a field-calibrated NMM accurately determined the change in soil water storage for the calculation of ET in three of the four soils.