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Abstract. Raats' [1971] steady state theory is here extended to provide an approximate analysis of the transient pattern of wetting around a point source. The assumption that a steady state regime prevails behind the wet front and the use of the Clothier [1984] and Philip [1984] theories on water movement along the streamlines allows an approximate transient solution for surface point source infiltration to be developed. This procedure was previously tested against absorption theory and laboratory experiments [Revol et al., 1995], but now this new analysis is evaluated via a field test. The vertical elongating influence of gravity is found to be well predicted. Application of this analysis to design purposes is also mentioned. The role of the macroscopic characteristic capillary length of unsaturated flow, Xc, is highlighted by this approximate solution. Finally, we propose a method that allows estimation of Xc from a point-source infiltration experiment. IntroductionThe lateral spread of water away from a dripper, compared to its depth of penetration, is determined by the relative importance of the soil's capillarity and its hydraulic conductivity. The ability to predict the dimensions of the wetted bulb is important, both to ensure efficient irrigation and to avoid the environmentally deleterious passage of irrigation water beyond the root zone. The main advantage of analytical solutions is their fewer parameters requirements and the generalization they offer in relating inputs to system response, which are essential for making general inferences and developing recommendations. Existing analytical theories for three-dimensional infiltration, which can be used for drip irrigation, either are applicable only to the early stages of wetting [

Abstract. Bresler [1978] proposed a procedure for drip irrigation design which is focused on the midpoint soil water pressure hc. We present a practical field test of this approach in order to evaluate the validity of the underlying assumptions. The simulated h c values were obtained from Raats' [1971] steady state theory for 32 points in the field where the hydraulic conductivity parameters K s and a were measured. The h c values were measured at the same locations during microirrigation of a maize crop. Measured hc's appear to be lower than the simulated ones, especially late in the season. The measured spatial variability in h c appeared to be higher than the simulated ones. This could well have been caused by root uptake activity, which is not considered in the analysis, as well as by the large but typical drippers spacing of d = 1.00 m. Thus the tensiometers could have been beyond the practical limit of wetting. Consequences for design and management are important. For design, even if a high h c value is chosen, there is no real guarantee that the wetting would be effective at the midpoint. For irrigation management, tensiometer placement too far from the dripper would lead to overirrigation, so for a large dripper spacing d, the midpoint placement is not judicious. IntroductionIt is well known that drip irrigation efficiency is critically dependent upon the soil's hydraulic properties. Nowadays, with increasing environmental concerns about water use and the recent development of new devices for measuring the soil's hydraulic properties, there is a requirement and opportunity to consider better the soil's properties in designing and operating drip irrigation systems. In this series of two papers we examine how the theory of point source infiltration can help in this task. This first paper deals with the soil water pressure at the point midway between drippers. The second paper IRevol et al.

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