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SOIL & WATER MANAGEMENT & CONSERVATION

Lysimetry versus Neutron Moisture Meter for Evapotranspiration Determination in Four Soils

USDA-ARS, Conservation and Production Research Lab., P.O. Drawer 10, Bushland, TX 79012

^* Corresponding author (Judy.Tolk{at}ars.usda.gov).

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 ET_NMM, vs. using weighing lysimeters (ET_LYS) 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 ET_NMM was 4% of the ET_LYS for the clay loam and sandy loam soils, but ET_NMM was 8% less than ET_LYS in the fine sand due to errors created by the timing of drainage and NMM measurements. At ET_LYS amounts <50 mm, the difference between ET_LYS and ET_NMM for individual measurement intervals could be as much as 28 mm and the average ET_NMM/ET_LYS ratios as much as 1.20. Beyond 100 mm of ET_LYS, the average ET_NMM/ET_LYS 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.

Abbreviations: ET, evapotranspiration • ET_LYS, ET with change in soil water storage determined using lysimetry • ET_NMM, ET with change in soil water storage determined using the neutron moisture meter • NMM, neutron moisture meter