Temporal variability in soil hydraulic properties under drip irrigation
Introduction
Drip irrigation has become quite common thanks to its great potential to use less water and to localize chemical applications, thereby enhancing the efficiency of irrigation and fertilization and reducing the risk of pollution. However, these objectives can only be achieved if the irrigation system is correctly designed (e.g. emitter discharge rate, emitter spacing, tape lateral spacing, diameter and length of the lateral system) and well managed (e.g. irrigation scheduling and fertilization strategy) for any given set of soil, crop and climatic conditions.
In contrast to surface or sprinkler systems, the frequency of the water application under drip irrigation is high. This means the infiltration period is a very important stage of the irrigation cycle (Rawlins, 1973). A good knowledge of the soil hydraulic properties involved in the multidirectional infiltration process during the course of this cycle is required to optimize water applications. The ability to estimate the dimensions of the wetting bulb i.e., water extending laterally and vertically away from an emitter is an important criterion for the design of drip systems to ensure efficient irrigation and to avoid the movement of water beyond the root zone (Bresler, 1978, Zur et al., 1994, Zur, 1996, Revol et al., 1997). Because analytical models provide a rapid way of determining the position of the wetting front (Revol et al., 1997, Cook et al., 2003, Thorburn et al., 2003), researchers have tried to develop a simple model to describe the soil wetting pattern with micro-irrigation systems. Schwartzman and Zur (1986) developed a simplified semi-empirical model of wetted soil geometry with surface trickle irrigation, which depends on specific parameters i.e., soil type (saturated hydraulic conductivity), emitter discharge per unit length of laterals, and total amount of water in the soil. Al-Qinna and Abu-Awwad (2001) estimated an exponential function with a water application rate to describe the horizontal width and the vertical depth of the advance of the wetting front. In their field study, Revol et al. (1997) found that the infiltration solutions of Philip (1984) provided good estimations of the radial (r) and vertical (z) distance of the wetted zone from the water sources. Warrick (2003) reviewed many analytical solutions describing water infiltration from point and line sources.
In a particular soil-water-plant system and climatic conditions, the transport properties of the soil surface layer can change during the growing season. This temporal variation is likely due to modifications in surface soil conditions resulting from tillage practices (Mohanty et al., 1996, Cameira et al., 2003), and to the effects of the rooting system (Shirmohammadi and Skaggs, 1984, Rasse et al., 2000, Iqbal et al., 2005). Wetting and drying cycles and the irrigation system can also alter the soil structure. Hydrodynamic behavior is consequently affected primarily by the current state of the soil structure, as well as its texture (Mapa et al., 1986, Messing and Jarvis, 1993, Angulo-Jaramillo et al., 1997, Cameira et al., 2003, Mailhol et al., 2005). However, the above-mentioned studies dealt with traditional irrigation systems that supply large amounts of water to the soil system at low frequencies. Only Mapa et al. (1986) addressed the effects on soil hydraulics of the wetting and drying cycles caused by drip irrigation following tillage. These authors found that soil hydraulic properties changed significantly after only one wetting/drying cycle in a silty clay loam and in a clay loam. However, in their study, each wetting/drying cycle included wetting (18 h of irrigation) followed by drying (7–10 days). In our opinion, this irrigation schedule thus more resembled that of traditional irrigation systems. To our knowledge, no study has tried to identify temporal variations in soil hydraulic properties during a cropping season under high-frequency water application.
The objectives of the present study were (i) to characterize temporal variability of soil hydraulic properties due to changes in soil structure under high-frequency drip irrigation, using the Beerkan infiltration method, and (ii) to illustrate the effects of temporal variability on the geometry of the wetting pattern generated by emitters, i.e., radius and depth, using the infiltration solution of Philip (1984).
Section snippets
Experimental site, soil and agricultural practices
Field experiments were conducted on a loamy soil containing an average of 43% sand, 40% silt and 17% clay in the plowed layer with a relatively small coefficient of variation. The experimental field is located at the Cemagref Experimental Station in Montpellier, France (43°40′ N, 3°50′ E) where there is a fully equipped meteorological station. In the 2006–2007 cultivation years, the field was plowed to a depth of 35 cm on November 15 with a moldboard plow. The seed bed (top 8 cm) was prepared
Soil hydraulic properties
Combining analysis of particle size distribution with modeling of the 3D infiltration experiments enabled us to fully determine the hydraulic parameters of the water retention and unsaturated hydraulic conductivity curves. Table 1 summarizes the statistical parameters of data sets of physical and hydraulic parameters.
The shape parameters of h(θ) and K(θ) varied little over time. This low variability is consistent with the assumption that the shape parameters mainly depend on soil texture (
Conclusions
In this study, the behavior of a loamy soil under drip irrigation was analyzed using the Beerkan infiltration method to identify the temporal variability of its hydraulic properties caused by high-frequency irrigation during a maize cropping season. Two different irrigation treatments, a full (FT) and a limited (LT), were investigated. Our results demonstrated that both soil porosity and hydraulic properties varied over time. Soil behavior can be divided into two separate stages. The first
Acknowledgement
The AEC of Syria is greatly acknowledged for the Ph.D. scholarship granted to Ibrahim Mubarak.
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