A Comparison Between Evapotranspiration Measurements Made by Weighing Lysimeter and Neutron Moisture Meter for Cotton Grown in Four Soils.

Knowledge of evapotranspiration (ET) is vital for the management of our fresh water 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 rainfall 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) and using weighing lysimeters.  Cotton was grown in 2006 and 2007 at the USDA-ARS, Bushland, TX in weighing lysimeters with a 0.75-m² surface area and 2.3-m depth containing monolithic soil cores, centrally located NMM access tubes, and drainage systems.  The texture of the four soils ranged from fine sand to clay loam.  The NMM was field-calibrated at the monolithic core collection sites. The change in soil water storage was measured daily with the weighing lysimeters, and at intervals ranging from 12 to 60 days using the NMM.  Drainage and rainfall were also measured.  The ET calculated using the NMM method tended to be within ±2% of the lysimeter ET for the finer textured soils, but NMM method underestimated ET by about 7% in the fine sand, probably due to errors created by the timing of the drainage and NMM measurements.  When all other soil water balance components are quantified, a field-calibrated NMM can accurately measure changes in soil water balance for the calculation of ET.