Some aspects of soil physics applicable to trickle irrigation : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science at Massey University

Abstract

Irrigation of crops is one of the more widely used techniques to increase yeilds. Trickle irrigation is one such method and is more suited to horticultural crops. In New Zealand, with horticulture assuming more importance, appropriate methods of design and operation of trickle irrigation systems are required. In this study a simple approximation to Wooding's solution for steady infiltration from a shallow ponded source, much like that found under trickle emitters is examined. This may aid in irrigation design and practice. The approximation also allowed for the development of a method to concurrently measure the saturated hydraulic conductivity and sorptivity from simple field infiltration measurements with a minimum of soil disturbance. Saturated hydraulic conductivities and sorptivities are of great use in soil water studies in general. A commercial trickle irrigation system was also examined to determine the suitability of such irrigation systems to particular soils, and to examine the present irrigation scheduling. The approximation to Wooding's solution was found to perform well in the field in many respects, particularly in determining steady ponded zone sizes. Ponded zone sizes are important in that they control the volume of soil wetted by irrigation to a large degree. Much of this agreement is due to the use of parameters determined by the simple field method developed from this theory. Sorptivities and saturated hydraulic conductivities obtained by this method were found to be more realistic for trickle irrigation than those determined by other existing methods. Systematic errors in these other methods, mainly soil disturbance and the concomitant creation of continuous flow paths for water, as well as soil smearing, are thought to be the main cause of this difference. Temporal and spatial variation in soil physical properties are however, found to hinder the use of soil physics theory in the field. Macropores (due to soil biological activity) were found to profoundly influence infiltration processes and soil-water distribution. These effects were particularly marked for the site with a commercial trickle irrigation system. Here the efficiency of the present system is thought to be low, and evidence indicates that irrigation was in excess of plant requirements. The utility of Wooding's solution, and the method to measure soil physical parameters developed from this, is further demonstrated in this orchard.