Effect of temporal variability in soil hydraulic properties on simulated water transfer under high-frequency drip irrigation
Introduction
Drip irrigation enables water and fertilizers to be applied at exact doses all over a field. If the drip irrigation systems are properly designed, managed and maintained for any given set of soil, crop and climatic conditions, drip irrigation improves the efficiency of water and fertilizer use and reduces the risk of pollution.
As the frequency of water application under drip irrigation is high, the infiltration period is a very important stage of the irrigation cycle (Rawlins, 1973). The extent to which water moves laterally and vertically away from a dripper is determined by the hydraulic properties of the soil (Philip, 1984, Revol et al., 1997). Estimating the dimensions of the wetting bulb is important for the design of drip systems to avoid water being transferred beyond the root zone (Bresler, 1978, Zur et al., 1994, Zur, 1996, Revol et al., 1997). Thus a good knowledge of the soil hydraulic properties involved in the multidirectional infiltration process over the course of the irrigation cycle is indispensable. Other factors such as emitter discharge rates, the amount water used, irrigation frequency, crop water uptake rates and root distribution patterns can influence the spatial distribution of soil water content and fertilizer concentrations.
The influence of these factors on moisture distribution patterns can be better understood using soil water simulation models. Numerical simulations are efficient tools to investigate optimal drip management practices (Meshkat et al., 1999, Assouline, 2002, Schmitz et al., 2002, Cote et al., 2003, Skaggs et al., 2004, Beggs et al., 2004, Li et al., 2005, Lazarovitch et al., 2005, Lazarovitch et al., 2007, Gardenas et al., 2005, Hanson et al., 2006, Hanson et al., 2008). Hydrus-2D (Simunek et al., 1999) is a well-known Windows-based computer software package for simulating water and solute transfer in variably saturated porous media. The accuracy of Hydrus-2D simulations of water infiltration and redistribution under drip irrigation has been evaluated for both bare and cultivated soils (Vrugt et al., 2001, Skaggs et al., 2004, Assouline et al., 2006, Arbat et al., 2008, Patel and Rajput, 2008). However, there have been few studies showing that numerical simulations of drip irrigation agree with long term field measurements under cultivated soil conditions, thus calling into question the value of the conclusions drawn from numerical simulations.
For a particular soil–water–plant system and given climatic conditions, the water transport properties of the soil surface layer may 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 effect of the root system (Shirmohammadi and Skaggs, 1984, Rasse et al., 2000, Iqbal et al., 2005). Wetting and drying cycles are also considered as the primary events causing structural transformations in tilled soils. Wetting of tilled soil by traditional irrigation or rainfall causes in aggregate breakdown (Shiel et al., 1988) resulting in reduced porosity, changes in soil pore size distribution and associated soil hydraulic properties (Collis-George and Greene, 1979, Mapa et al., 1986, Kemper et al., 1988, Cameira et al., 2003, Mubarak et al., submitted for publication, Mubarak et al., 2009). Modeling of temporal dynamics of soil structure after tillage is recently advanced thanks to the advances in statistical and parametric methods for expressing soil pore size distribution. Model results of pore size evolution have been used to predict temporal changes in soil hydraulic functions (Or, 1995, Ghezzehei and Or, 2000, Or et al., 2000, Leij et al., 2002b). According to these models, temporal changes in soil structure due to wetting–drying cycles occurred in the structural pore space only, whereas textural porosity was assumed stable with time. The saturated hydraulic conductivity that decreases after tillage and after total porosity loss is reflected by reduction in saturated water content with time (Or et al., 2000). However, further studies are needed to provide experimental validation of these models. Strudley et al. (2008) reported in their literature review of tillage effects on soil hydraulic properties in space and time, that “very little work has been done to measure the effects of irrigation on soil hydraulic properties. Theoretical advances are similarly limited (Or, 1996, Or et al., 2000, Leij et al., 2002a, Leij et al., 2002b, Or and Ghezzehei, 2002)”.
Similar to traditional irrigation systems, drip irrigation can also alter soil structure and the soil hydraulic behavior can be affected as a result. Mapa et al. (1986) experimentally studied the effects of wetting and drying caused by low frequency drip irrigation on soil hydraulics following tillage. These authors found that water retention characteristics and hydraulic conductivity changed significantly over the low-suction/high water content range after only one wetting/drying cycle in Molokai silty clay loam and Waialua clay loam in O’ahu, Hawaii. Mubarak et al. (submitted for publication) studied series of 3D in situ infiltration tests and ShC analysis carried out on a silty clay loam soil in two different structural conditions. Their results allowed to get insights on the relationship between the changes over time in soil hydraulic properties and in pore space during a drip irrigated cropping cycle. ShC analysis was an additional assessment of soil structure and stability. This assessment helped interpreting the results derived from the Beerkan infiltration method by a determination covering the whole range of pore dimensions with quantitative and descriptive distinction between textural porosity and structural porosity. The results of both methods showed that the large structural pore developed over time will increase saturated hydraulic conductivity and the effect of gravity, while the textural porosity behavior will remain constant over time due to the constant soil texture. Mubarak et al. (2009) intensively identified temporal variability in field soil hydraulic properties under high-frequency drip irrigation during a maize cropping season on a loamy soil in the south of France using the Beerkan infiltration method. Their results demonstrated that both soil porosity and hydraulic properties changed over time as a consequence of drip irrigation, alternative cycles of wetting and drying, biological activity, and root development. The temporal changes in soil behavior could be divided into two separate stages. The 1st 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 events, resulting in a decrease in both the saturated hydraulic conductivity Ks and the mean pore effective radius ξm as well as in an increase in capillary length αh. These hydraulic parameters attained their extreme values at the end of this stage. This behavior was explained by the “hydraulic” compaction of the surface soil following irrigation. Later in the season, i.e., during the 2nd stage, a gradual increase in both Ks and ξm and a gradual decrease in αh were observed when the effect of irrigation was overtaken by another phenomenon. The latter was put down to the effects of wetting and drying cycles associated with those of the root system, which could be asymmetric as a result of irrigation with only one drip line installed for every two plant rows. Some roots could grow and develop preferentially in the vicinity of emitters, creating new channels or continuity between existing pores. Another possible explanation mentioned was re-opening of the connection between structural pores due to the soil biological activity that is locally stimulated by permanent high humidity in the wetted zone of the soil. This work raised a question about the best time in a cropping season to evaluate the hydraulic properties of a soil to improve water management and to mitigate agro-environmental risks. Research was consequently needed to illustrate the effects of the temporal variability of soil hydraulic properties on moisture distribution patterns under drip irrigation. The objective of the present work was to decide whether or not the changes in the topsoil parameters identified in Mubarak et al. (2009) should be taken into account over the course of a cropping season under high-frequency drip irrigation by comparing soil water contents measured with both neutron probes and capacitance sensors placed in access tubes (EnviroSMART) with the results obtained with the Hydrus-2D model.
Section snippets
Field description and measurements
The study site is located at the Domain of Lavalette at the Cemagref Experimental Station in Montpellier, France (43°40′N, 3°50′E) where there is a fully equipped meteorological station (see Mubarak et al., 2009). The surface soil was plowed and then the 8 cm topsoil was ground into aggregates less than 2 mm in diameter. Maize (Pionner PR35Y65) was sown on 24 April 2007 with a row spacing of 0.75 m and a plant density of 100,000 ha−1. The drip irrigation system was installed at the beginning of
Conversion of topsoil hydraulic properties
The fitted water retention and unsaturated hydraulic conductivity curves agreed extremely well with the data generated using topsoil hydraulic parameters determined by the Beerkan infiltration method (Fig. 6). The performance criterion of parameter calibration, R2, using the RETC code for the three cases of topsoil hydraulic parameters is typically high and ranges between 0.941 and 0.993. This shows that the method used to convert the soil hydraulic parameters was efficient in finding
Conclusions
The Hydrus-2D code was used to study the effect of temporal variability in the hydraulic properties of a loamy soil during a maize cropping cycle on water transfer under daily drip irrigation. Simulated soil water contents were compared to those continually measured in the field using capacitance sensors (EnviroSMART) and neutron probes. Three sets of soil hydraulic parameters according to three different structural conditions of the surface soil layer as identified in Mubarak et al. (2009)
Acknowledgements
The AEC of Syria is greatly acknowledged for the PhD scholarship granted to Ibrahim Mubarak. The authors are grateful to Mr. Jean Paul LAURENT (LTHE Grenoble University, France) for providing the capacitance sensors (EnviroSMART). The authors would also like to thank Mr. Patrick ROSIQUE and Mr. Laurent DELAGE for their assistance in the field measurements.
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