Use of the Neutron Probe in the Lowveld of Zimbabwe

ncutron probe use in the Lowveld is repor-ted. This assessAs part of a watcr management project initiated by the Zimbabwe Sugar Association Experiment Station (ZSAES), three large sugar estates and the ZSAES have imported a commercially available irrigation scheduling package which is based on neutron probe measurements of soil water. Analysis of selected data collected using the package highlighted inefficiencies in irrigation, viz. inflated drained upper limits (DUL) or 'full' points, irrigation water applications that were too large and too frequent when determined using a water budget and inflated to cater for assumed low efficiencies, and too short a 'drying-off' period. At the sites discussed in the paper, potential savings in water of approximent is set in context by providing a broad perspective of issues and tools for the management of irrigation watcr, especially with regard to the potential role of neutron probes. Data from two furrow irrigation sites have been selected to focus discussion on various key topics. ~ i z : draincd upper limits (DUL) or 'full' points 'refill' points, estimated daily crop water use neutron probc calibration, irrigation application efficiencies irrigation system performance evaluation. Irrigation water management mately 200 mm/ha are evident and the inefficiency i n IrrigaThe context of neutron probe use in irrigation water mantion water applications is the likely cause of yield reductions agement is illustrated and discussed with reference to Figure i n the order of 15 tlha. The amounts of water that a soil could 1 , with thc focus on the 'Information' aspects of irrigation 'store' just prior to an irrigation, and the magnitude of the scheduling. When scheduling irrigation water applications, irrigation, were the major determinants of application effithe status of soil water is most often used as the irrigation ciencies, even with furrow irrigation which is often assumed decision variable, viz. to determine when and how much to be self-compensating through a wetter soil having a more water to apply. 'The soil water status can be measured directrapid advance front. Procedures which accommodate typical ly, for examplc, using a neutron probe or some other instruestate land husbandry arc described to perform a practical ment, or i t can be estimated using indirect means, which usucheck on the neutron probe's calibration, and to determine ally involvcs a water budget. In most water budgets, an estian in sitit drainage curve and obtain an objective 'full' point mate of atmospheric evaporative demand (AED) is used which is representative of in-field conditions. together with informatton on the crop's development/canopy status to determine potential water withdrawals. These estiIntroduction mates can be relatively simple: the crop status represented by a time dependent crop coefAs part of an effort to improve water management, three ficicnt and AED represented by evaporation from an Alarge sugar estates in the Lowveld of Zimbabwe and the Pan Zimbabwe Sugar Association Experiment Station (ZSAES) have imported a commercially available irrigation scheduling package which is based on neutron probe measurements of soil water. The packagc encompasses all aspects of neutron probe use for irrigation scheduling, including hardware, software, calibration relationships, and guidelines for opcrating the neutron probe and for the interpretation of data. The neutron probe was selected from a range of soil water measurement devices because its usefulness has been proven (Cull, 1992) and, when compared with instruments that are left in the field, potential problems of theft are reduced. or relatively complex: the crop's status being determincd by more physically or conceptually explicit relationships, for example, by relating canopy development to thelmal timc in a crop model AED represented by more physically explicit combinations of weather mcasurements, for example, the PenmanMonteith evaporation equation the interaction bctween AED and the crop rcpresented by soil dependent pattcriis of potential and actual root water uptake, as, for example, are used at different levels of complexity i n thc ACRU (Lecler and Schulze, 1995) or The suppliers of the neutron probe package commissioned CANEGROIDSSAT ('personal communication) models. thc system in the Lowvcld and trained the operators. They Arguments for measuring thc soil waterstatus directly are: also made a subsequent visit to assess the situation and prewhether a simple or a complex water budget is used, there sented their findings at a Zimbabwe Sugar Seminar (Lanscr, arc many unknowns in estimating evaporation from a 1998). In this paper, a rusther, independent assess~nenl of cropped surface and these can lead 10 significant errors in 16 Proc S A,fr S ~ l g Technol Ass (1999) 73 Use of the neiltrorl probe it? the lowveld of Zimbabwe NL Leclet; C Nyati & M Sheppard