Soil-dependent wetting from trickle emitters: implications for system design and management

For trickle irrigation systems to deliver improved water- and nutrient-use efficiency, distance between emitters and emitter flow rates must be matched to the soil's wetting characteristics and the amount and timing of water to be supplied to the crop. Broad soil texture ranges (e.g. sand, loam, clay) are usually the only information related to soil wetting used in trickle system designs. In this study, dimensions of wetted soil were calculated from hydraulic properties of 29 soils covering a wide range of textures and soil hydraulic properties to assess the impact of soil texture and/or type on soil wetting patterns. The soils came from two groups that differed in the extent to which hydraulic properties depended on soil texture. Vertical and radial distances to the wetting front from both surface and buried emitters were calculated for conditions commonly associated with daily irrigation applications in a widely spaced row crop (sugarcane) and horticultural crops. In the first group of soils, which had least expression of field structure, the wetted volume became more spherical (i.e. the wetted radius increased relative to the depth of wetting below the emitter) with increasing clay content, as is commonly accepted. However, in the second group of soils in which field structure was preserved, there was no such relationship between wetted dimensions and texture. For example, five soils with the same texture had as great a variation in wetting pattern, as did all 11 soils in the first group, indicating the considerable impact of field structure on wetting patterns. The implications of the results for system design and management were illustrated by comparing current recommendations for trickle irrigation systems in coastal northeastern Australia with the calculated wetted dimensions. The results suggest that (1) emitter spacings recommended for sugarcane are generally too large to allow complete wetting between emitters, and (2) the depth of wetting may be greater than the active root zone for both sugarcane and small crops in many soils, resulting in losses of water and chemicals below the root zone. We conclude that texture is an unreliable predictor of wetting and there is no basis for adopting different dripper spacing in soils of different textures in the absence of site-specific information on soil wetting. Such information is crucial for the design of efficient trickle irrigation systems.