Aspects of the water balance of an oats crop grown on a layered soil : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
The increasing pressure on our water resources, for irrigation in particular, has resulted in a growing awareness of the importance of water balance studies. In this thesis three aspects of the field water balance are investigated; evapotranspiration (ET) from well-watered crops, the upper limit of soil water storage in the field, and drainage. Daily ET values, measured by the Bowen ratio-energy balance method, are presented for an oats crop grown in winter and also for a number of summer crops, all of which were well-watered, ET measurements were also made over longer periods using a drainage lysimeter. It was found that the Penman, and Priestley and Taylor ET estimation procedures predicted ET with an accuracy of 15-20% and 8% for daily and weekly periods, respectively. The Priestley and Taylor method is simpler to use but requires an empirical constant to relate the 'equilibrium ET' to ET. This constant was found to be 1.21 for winter, spring and summer over a range of crops in the Manawatu. Net radiation data on a daylight basis were used to evaluate this constant, as seasonal variations in the constant were introduced when 24-hour data were used. Also it is easier to empirically estimate daylight than 24-hour net radiation. Long term ET estimates using the Priestley and Taylor method with net radiation calculated from incoming solar radiation, were in reasonable agreement with the drainage lysimeter measurements of ET for the oats crop. A theoretical development is presented that describes water retention in soils underlain by a coarse-textured stratum. This development accounts for the physical character of the overlying soil, the depth to the coarse layer, and the coarseness of the underlay. Field data are presented for the Manawatu fine sandy loam, a soil with a coarse-textured layer at 90 cm. For this soil the layering resulted in an additional 55 mm of water storage at the cessation of drainage, an increase of 31% over a similar hypothetical soil with the coarse stratum absent. Drainage from a permeable soil underlain by a coarse-textured layer is investigated. Simplified theory is used to develop a model relating the drainage flux at the base of the soil to the water stored in the overlying soil. Despite significant hysteresis in both the water retentivity curve of the overlying soil and the hydraulic conductivity-pressure potential relationship of the coarse layer, hysteresis had little effect on the storage-flux relation. The model simulated both the field drainage in the Manawatu fine sandy loam measured by a lysimeter, and field profile water storage found by neutron probe moisture measurements. The model indicates that only simple field measurements are needed to find the storage-flux relationship. The components of the water balance of an autumn-sown oats crop grown in the Manawatu are resolved. Drainage loss was found to constitute 60% of the rainfall, with the remaining amount being lost as ET.