An Exploratory Analysis of Soil Profile Water Content and Estimated ET with Neutron Moisture Meters

Soil water balance is a sound alternative to lysimetry for the measurement of crop water use, or evapotranspiration (ET). This involves measuring the components of the water balance, i.e., precipitation, irrigation, deep percolation, runoff, and change in soil profile water storage, for a control volume defined by soil profile of given depth. The reliability of ET estimates from the soil water balance method depends on the measurement or estimation accuracy of the above variables. Deep percolation (or upward flow) is the most difficult variable to quantify and accounts for most mass balance errors in estimating ET. This study investigated variations in soil profile water content, and calculated ET, between adjacent small plots in a crop season and in the same plot during two seasons. The study was conducted in drip irrigated cotton plots under a range of irrigation regimes during 2004 to 2006 cropping seasons. Neutron probe measurements were made for each 0.15 m layer in the soil profile to the bottom of the access tube at 1.8 m, except the top 0.15 m which was measured gravimetrically. No shallow water tables existed at the study site, thus only deep percolation was of concern. No independent verification of deep percolation was available in this study. Considerable spatial and temporal variations in measured soil profile water and estimated percolation occurred. Variations were partly soil and partly irrigation induced, with the latter having a greater impact on estimated ET. The installation of the access tubes at relatively deep depths, i.e., 1.8 m, was found very useful, providing sufficient depth for detection of potential deep percolation below the root zone. Presentation will elaborate on the usefulness of the exploratory analysis of the detailed soil profile water measurements and the key findings.