Soil Physics
Soil Water and Salinity Distribution under Different Treatments of Drip Irrigation Tarek Selim*
Civil Engineering Department Faculty of Engineering Port Said University Port Said, Egypt and Department of Water Resources Engineering Lund University Box 118 221 00 Lund, Sweden
Fethi Bouksila
National Institute for Research in Rural Engineering, Water, and Forests Box 10 2080 Ariana, Tunisia
Ronny Berndtsson
Department of Water Resources Engineering Lund University Box 118 221 00 Lund, Sweden and Center for Middle Eastern Studies Lund University Box 201 221 00 Lund, Sweden
Magnus Persson
Department of Water Resources Engineering Lund University Box 118 221 00 Lund, Sweden
In this study, field experiments and numerical simulations for different drip irrigation treatments in a sandy loam soil were conducted to investigate soil water and salinity distribution as well as dye infiltration patterns. Three treatments, surface drip irrigation without and with plastic mulch (T1 and T2, respectively) and subsurface drip irrigation (T3), were used. Daily and bi-weekly irrigation regimes were used for each treatment. After completion of each irrigation treatment, horizontal soil sections were dug with 10 cm intervals. Dye patterns were photographed using a digital camera and soil water and pore water electric conductivity were measured by a WET-sensor. Experimental results revealed that maximum dye infiltration depth and maximum dye coverage volume occurred during the bi-weekly irrigation regime and in T3. Daily irrigation regime kept the top soil layer moist with adequate amount of soil water as compared to the bi-weekly irrigation. Moreover, T2 provided higher soil water content within the soil domain as compared to other treatments. The simulation results demonstrated that model prediction for soil moisture distribution within the flow domain was excellent. Furthermore, T2 and daily irrigation showed lower salinity levels in the flow domain as compared to other irrigation treatments and regimes. In sum, mulching treatment with daily irrigation regime is recommended for arid areas over other drip irrigation treatments and regimes. In addition, HYDRUS-2D/3D can be used as a fast and cost effective assessment tool for water flow and salt movement for sites having similar soil conditions. Abbreviations: ECa, bulk electrical conductivity; ECp, pore electrical conductivity; ET0, evapotranspiration; FDR, frequency domain reflectometry; Ka, apparent dielectric constant; Ks, saturated hydraulic conductivity; T1, surface drip irrigation without plastic mulch; T2, drip irrigation with plastic mulch; T3, subsurface drip irrigation; T1D, daily surface drip irrigation without plastic mulch; T3D, daily subsurface drip irrigation with dripper 10 cm below the soil surface; T1W, bi-weekly surface drip irrigation without plastic mulch; T2W, bi-weekly surface drip irrigation with plastic mulch; T3W, bi-weekly subsurface drip irrigation with dripper 10 cm below the soil surface.
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rowing populations and socioeconomic circumstances force arid and semiarid countries to use all available water resources including low quality water (i.e., brackish water, drainage water, and treated waste water) to develop their agricultural production. Furthermore, it is necessary to reduce the volume of drainage water requiring disposal or treatment that in turn could deteriorate the environment. At the same time, water quality as well as agricultural and irrigation practices are considered the major factors affecting crop productivity and soil sustainability. In arid and semiarid countries, the use of brackish irrigation water is often associated by soil salinization risk due to mismanagement. In case of shallow ground water, excessive irrigation can lead to an increase in ground water table and salinity levels by upward salt flux (e.g., Hamed et al., 2008). Also, it might cause water logging and soil salinization (e.g., Bouksila et al., 2013). World Soil Sci. Soc. Am. J. 77:1144–1156 doi:10.2136/sssaj2012.0304 Received 17 Sept. 2012. *Corresponding author (
[email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Soil Science Society of America Journal
Resources Institute (1998) stated that poor agricultural and irrigation practices had caused degradation by about 38% of worldwide crop land by 1990. Also, land degradation has continued since 1990 at a rate of 5 to 6 million ha annually. According to this rate, the irrigated land will be out of production in 140 yr (Bouksila, 2011). Tunisia is a semiarid to arid country, about 50% of Tunisia’s water resources have a salinity level of more than 1.5 dS m–1, and it exceeds 3 dS m–1 in 47 and 67% of the shallow and deep ground water aquifers, respectively. Irrigation consumes about 81% of Tunisia’s water resources. Due to low water quality and improper agricultural and irrigation practices, around 50% of the irrigated areas in Tunisia nowadays suffer from soil degradation (DGACTA, 2007). To stop this disastrous trend, enhancing the use of proper irrigation methods with better irrigation management is needed to control the salinity levels and water consumption. In addition, monitoring and assessment of the potential threats to ground water due to the extensive use of brackish irrigation water, fertilizers, and pesticides should be conducted. Among many irrigation methods, drip irrigation is considered a suitable technique in arid and semiarid countries to irrigate row crops (e.g., vegetables and fruit trees). Through drip irrigation, water and fertilizers can be adequately applied to the root zone, meanwhile, reducing evaporation and deep percolation (e.g., Yohannes and Tadesse, 1998). Larger crop production and better water use efficiency are usually achieved simultaneously with drip irrigation as compared to other surface irrigation methods (e.g., Pruitt et al., 1989; Yohannes and Tadesse, 1998; Colaizzi et al., 2006; Liao et al., 2008). In addition, higher salinity levels in the irrigation water can be tolerated while maintaining yields comparable to other irrigation methods (e.g., Rhoades et al., 1992; Tingwu et al., 2003). In irrigated districts in Tunisia, farmers prefer drip irrigation as compared to sprinkler irrigation to avoid the problem of leaf burns (Mekki and Bouksila, 2008). Dye experiments accompanied with soil salinity mapping can be used to produce risk mapping for soil and ground water salinization and contamination under given irrigation treatments. Using dye is a powerful method for capturing the spatial flow patterns through field soil under different irrigation methods. Through these patterns the prospective threat to soil and ground water contaminations due to pesticides and herbicides can be assessed. Although Brilliant Blue (BB) dye is not suitable to simulate water movement, it mimics the behavior of organic compounds such as some pesticides, herbicides, and organic fertilizers (e.g., Kasteel et al., 2002). The dye as a tracer has been widely used in many soil studies (e.g., Ewing and Horton, 1999; Yasuda et al., 2001; Abou Lila et al., 2005; Hamed et al., 2008; Nobles et al., 2010). Despite that dye experiments supply information about flow patterns at a much finer resolution as compared to other techniques, the use still raises some problems. It is time consuming and labor intensive since it requires detailed excavation of the site and photographing consecutive soil sections (e.g., Hamed, 2008). Nevertheless, the detailed excavation facilitates extensive in situ measurements of soil water content and pore water www.soils.org/publications/sssaj
electrical conductivity at many locations within the soil domain by, for example, frequency domain reflectometry (FDR) sensors (e.g., sigma probe, WET sensor; Öhrström et al., 2004). With a particular measuring scheme, spatial visualization and mapping of soil water and salinity distribution under given irrigation treatment can be conducted. The WET sensor is capable of measuring soil water content and soil solution electrical conductivity. Although FDR sensors are cheaper than other electromagnetic devices (i.e., time domain reflectometry), portable, and easy to use, its readings are affected by soil texture and salinity levels (Persson, 2002; Hamed et al., 2006; Bouksila et al., 2008). Owing to the limitations of the FDR sensors under some field conditions and as the infiltration experiments do not show the flow dynamics, a quantitative presentation of the flow dynamics and salt movement through a calibrated and validated simulation model is useful. In addition, combining a modeling approach with field data creates a rapid and cost effective assessment of water and salt movement under drip irrigation treatments with brackish water. Most drip irrigation studies conducted in Tunisia have been site and crop specific (e.g., Kahlaoui et al., 2011; Douh and Boujelben, 2011). There is a clear lack in dynamic modeling approach to assess such systems and to reduce the dependence on experimental research at sites having similar soil conditions. Among many simulation models, the numerical model, HYDRUS-2D/3D, (Simunek et al., 2008) is considered an effective and precise tool for simulating water flow and solute transport under different irrigation techniques including drip irrigation. It has been widely used to simulate hypothetical problems as well as verifying experimental data with and without crops (e.g., Abou Lila et al., 2012; Selim et al., 2012; Bufon et al., 2011; Phogat et al., 2012; Ajdary et al., 2007; Gardenas et al., 2005; Skaggs et al., 2004; Assouline, 2002). In addition, it provides more precise estimation of water movement under drip irrigation compared to analytical and empirical models (e.g., Kandelous and Simunek, 2010). In view of the above, the specific objectives of the present study were to (i) investigate the effect of drip irrigation treatment and irrigation regime on soil water and salinity distribution and (ii) assess the potential ground water contamination risk under selected irrigation treatments and regimes. Additional objectives of the current study were to (i) investigate the efficiency of the WET sensor for measuring soil solution electric conductivity in sandy loam soil and (ii) assess the possibility of using HYDRUS2D/3D model as an alternate technique rather than field experiments for studying water and salt movements for specific sites having similar soil conditions.
METHODS AND MATERIALS Site Description
Field experiments were performed in April 2012 (entire month) at the research station of Souhil River, Nabeul, 70 km southeast of Tunis. The climate at Nabeul is Mediterranean semiarid with average annual precipitation and potential 1145
evapotranspiration of 450 and 1370 mm, respectively. During the experimental period, climatic data were recorded at a weather station 500 m from the experimental plot where daily minimum and maximum temperature ranged from 7 to 15 and 18 to 25°C, respectively, and the evaporation rate was measured using a class A pan. Average evaporation rate was 3.40 mm d–1 and a total rainfall of 37 mm was received during the experimental period. Approximately all rainfall events occurred during the middle third of the experimental period. The soil at the experimental site is classified as sandy loam (see Table 1) and the ground water table is suited 4 m below the soil surface. The experimental site was used for cultivation since 2010 until the end of 2011 (i.e., 4 mo before the start of experiments). Surface drip irrigation is the common irrigation method used for watering crops in this field. According to measurements conducted directly before starting the experimental work, it was estimated that the initial soil moisture content was uniform in the horizontal direction and changed linearly with depth from 0.07 m3 m–3 at the soil surface to 0.10 m3 m–3 at 100-cm depth. In addition, soil salinity levels were
