Elsevier

Geoderma

Volume 150, Issues 1–2, 15 April 2009, Pages 158-165
Geoderma

Temporal variability in soil hydraulic properties under drip irrigation

https://doi.org/10.1016/j.geoderma.2009.01.022Get rights and content

Abstract

Predicting soil hydraulic properties and understanding their temporal variability during the irrigated cropping season are required to mitigate agro-environmental risks. This paper reports field measurements of soil hydraulic properties under two drip irrigation treatments, full (FT) and limited (LT). The objective was to identify the temporal variability of the hydraulic properties of field soil under high-frequency water application during a maize cropping season. Soil hydraulics were characterized using the Beerkan infiltration method. Seven sets of infiltration measurements were taken for each irrigation treatment during the cropping season between June and September 2007. The first set was measured two weeks before the first irrigation event. The results demonstrated that both soil porosity and hydraulic properties changed over time. These temporal changes occurred in two distinct stages. The first stage lasted from the first irrigation event until the root system was well established. During this stage, soil porosity was significantly affected by the first irrigation event, resulting in a decrease in both the saturated hydraulic conductivity Ks and the mean pore effective radius ξm and in an increase in capillary length αh. These hydraulic parameters reached their extreme values at the end of this stage. This behavior was explained by the “hydraulic” compaction of the surface soil following irrigation. During the second stage, there was a gradual increase in both Ks and ξm and a gradual decrease in αh when the effect of irrigation was overtaken by other phenomena. The latter was put down to the effects of wetting and drying cycles, soil biological activity and the effects of the root system, which could be asymmetric as a result of irrigation with only one drip line installed for every two plant rows.

The processes that affected soil hydraulic properties in the two irrigation treatments were similar. No significant change in ξm and αh was observed between FT and LT. However, as a result of daily wetting and drying cycles, which were strongest in LT, the soil in this treatment was found to be more conductive than that of FT. This showed that most of the changes in pore-size distribution occurred in the larger fraction of pores.

The impact of these temporal changes on the dimensions of the wetting bulb was studied using a simplified modeling approach. Our results showed that there were marked differences in the computed width and depth of wetting bulb when model input parameters measured before and after irrigation were used. A temporal increase in capillary length led to a more horizontally elongated wetting bulb. This could improve both watering and fertilization of the root zone and reduce losses due to deep percolation. As a practical result of this study, in order to mitigate agro-environmental risks we recommend applying fertilizers after the restructuration of tilled soil. Further studies using improved models accounting for temporal changes in soil hydraulic properties are needed.

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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