Author's personal copy Measured and simulated soil wetting patterns under porous clay pipe sub-surface irrigation A.A. Siyal a, *, T.H. Skaggs b a Department of Land and Water Management, Sindh Agriculture University, Tandojam, Pakistan b U.S. Salinity Laboratory, USDA-ARS, 450 W. Big Springs Rd., Riverside, CA 92507, USA 1. Introduction Improving agricultural water use efficiency is vitally important in many parts of the world that have limited water resources. Sub- surface irrigation, in which water is applied below the soil surface, can help conserve water by reducing evaporative water losses in agricultural systems. Sub-surface irrigation has been practiced in various forms since ancient times, including pitcher or pot irrigation (e.g. Bainbridge, 2001; Siyal et al., submitted for publication) and perforated or porous clay pipe irrigation (e.g. Ashrafi et al., 2002; Qiaosheng et al., 2007). The development of plastic micro-irrigation technology in the last century led to increased use of sub-surface irrigation. Today, sub-surface micro- irrigation is used throughout the world to irrigate field crops, vegetables, and fruits (Camp, 1998). However, in many parts of the world, plastic drip tubing and emitters are cost-prohibitive, and traditional methods such as clay pipe irrigation remain an important technique for irrigation and water conservation. In traditional sub-surface pipe irrigation systems perforated or porous pipes are buried in the soil. Water seeps from the pipe into the soil and spreads out in the root zone due to capillary forces. Batchelor et al. (1996), Hegazi (1998), and Bainbridge (2001) found that traditional sub-surface irrigation methods have high water use efficiency, i.e., high crop production per amount of applied water. The efficiency may be affected by the water application rate and by system design parameters such as the size, depth, and spacing of pipes, etc., which determine the extent of deep percolation water losses and soil saturation problems. Also, evaporation losses are minimized when the wetting front is kept below the soil surface. Hence, an ability to predict the geometry and moisture distribution of the wetted zone for different soils, pipe compositions, and system designs can be very useful for developing guidelines and criteria for optimizing the performance of traditional sub-surface irrigation systems (e.g. Zur, 1996). Soil wetting patterns under surface and sub-surface micro- irrigation have been measured and/or analyzed theoretically by a number of authors, including Bresler (1978), Assouline (2002), Cote et al. (2003), Skaggs et al. (2004), Ga ¨ rdena ¨ s et al. (2005), Singh et al. (2006), Wang et al. (2006), and Lazarovitch et al. (2007), to name only a few. Among these studies, several of the more recent ones (e.g. Assouline, 2002; Cote et al., 2003; Skaggs et al., 2004; Ga ¨ rdena ¨ s et al., 2005) have used HYDRUS-2D (S ˇ imu ˚ nek et al., 1999) to simulate soil wetting. For example, Skaggs et al. (2004) demonstrated that HYDRUS-2D simulations of drip irrigation were in agreement with detailed field measurements. Nevertheless, not much work has been done focusing specifically on traditional sub-surface systems. Analyzing traditional systems differs in that it is generally necessary Agricultural Water Management 96 (2009) 893–904 ARTICLE INFO Article history: Received 28 July 2008 Accepted 21 November 2008 Keywords: Porous clay pipe Sub-surface irrigation Micro-irrigation Soil wetting Hydraulic conductivity HYDRUS ABSTRACT Sub-surface irrigation with porous clay pipe can be an efficient, water saving method of irrigation for many less developed arid and semi-arid regions. Maximizing the efficiency of clay pipe irrigation requires guidelines and criteria for system design and operation. In this study, experimental and simulated (with HYDRUS (2D/3D)) soil wetting patterns were investigated for sub-surface pipe systems operating at different water pressures. Predictions of the soil water content made with HYDRUS were found to be in good agreement (R 2 = 0.98) with the observed data. Additional simulations with HYDRUS were used to study the effects of various design parameters on soil wetting. Increasing the system pressure increased the size of the wetted zone. The installation depth affects the recommended lateral spacing as well as the amount of evaporative water loss. For a given water application, the potential rate of surface evaporation affected the shape of the wetted region only minimally. Soil texture, due to its connection to soil hydraulic conductivity and water retention, has a larger impact on the wetting geometry. In general, greater horizontal spreading occurs in fine texture soils, or in the case of layered soils, in the finer textured layers. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +92 343 835 8031. E-mail addresses: siyal@yahoo.com (A.A. Siyal), Todd.Skaggs@ars.usda.gov (T.H. Skaggs). Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat 0378-3774/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2008.11.013