Constant Shallow Water Table Depth with Saline Ground Water

Full Length Research Paper

Assessment of evaporation and salt accumulation in bare soil: Constant shallow water table depth with saline ground water

Jalili, S.*, Moazed, H., Boroomand Nasab, S and Naseri, A. A.

Faculty of Water Sciences, Shahid Chamran University, Ahvaz, I. R. Iran.

Accepted 9 May, 2011

Salinization of soil is a major problem in arid and semi-arid regions with saline shallow water table. This is influenced by climate, soil type, crop, irrigation water quality and management practice, depth of water table and salinity of the water table. The objective of this study was the assessment of evaporation and salinization of bare soil profile with various water table depths including 300, 500 and 800 mm by saline groundwater (3 g NaCl/L + 3.5 g CaCl2/l). In this research we were monitoring amount of evaporation from bare soil, and observing profiles of soil water and salinity over periods of up to 80 days. The experiments started on April, 10th, 2010. Results showed that, salts mainly accumulated in the soil surface and volume of evaporation from bare soil in lower water table depth (D = 300 mm) is greater than other lysimeters (D = 500 and 800 mm), and salt accumulation in this lysimeter was higher than others. After 80 days, amount of salinity (ECe) in soil surface was 83.84, 65.76 and 24.05 dS/m, for D = 300, 500 and 800 mm, respectively.

Key words: Evaporation, salinity, water table.

INTRODUCTION

Salinization of soil is a major problem in arid and semi- flux can be beneficial to agriculture from the standpoint of arid areas with saline shallow water table. Salinization is meeting crop water requirements, but on the other hand, influenced by climate, soil type, crop, irrigation water salinity in the water can lead to crop damage and soil quality and management practice, depth of water table degradation. Young et al. (2007) reported that upward and salinity of water table. About 30% of arable land in flux was limited by the atmospheric evaporative potential Iran is saline where salinization of soil is primarily caused when the water table was shallow. With deeper water by capillary rise from saline shallow water table tables, soil physical properties limited upward flow. (Jorenush and Sepaskhah, 2003). Shallow water table Gardner (1958) also describes the contribution of the conditions may be found quite extensively in arid and vapor phase to overall evaporation. It was concluded that semi-arid environments. Their existence in irrigated areas subsurface vapor movement would seldom exceed 20% is often associated with problems of irrigation-induced of maximum liquid transport and would usually be much salinisation. However, they may exist also in non-irrigated less. More recent research has reinforced this conclusion environments, as, for example in playa areas and on the by finding that liquid water movement deeper in the soil fringes of rivers and lakes. Water that moves upwards limits total evaporation on daily or greater time scales through capillary rise from a shallow water table can (Saravanapavan and Salvucci, 2000). In this regard, the enter the atmosphere through plant transpiration or direct term "critical water table depth" is often mentioned and evaporation from bare soil. On the one hand, this upward considered as the level above which water rising by capillarity will cause salinization of the arable soil horizons. Kovda (1973) relates this critical depth to the salt content of the groundwater in arid zone and areas *Corresponding author. E-mail: [email protected]. provided with irrigation and drainage as, respectively, Jalili et al. 6069

Table 1. The selected chemical and physical properteis of original soil.

Soil properties Amount Ca2+ 10 meq/L Mg2+ 3 meq/L Na+ 24.8 meq/L K+ 1.41 meq/L Cation exchange capacity 31.34 meq/100 g soil pH 7.7

ECe 2.55 Organic material 0.74% Texture Clay loam

being 2 to 2.5 when the salt concentration is 10 to 15 g/L, water and salt from a shallow saline water table in field and 1 to 1.5 m for a less mineralized groundwater, 1 to 2 condition evaporation. In the author's opinion, the g/L. For bare soils, the critical depth coincides with the effective control of soil salinity and also of salinity in depth from soil surface to the water table but for cropped Shallow water table conditions requires the following: i) soils, this depth should be taken below an estimated knowledge of the magnitude, extent and distribution of active root zone. soil salinity and, ii) knowledge of the changes and trends The significance of the depth to water table comes from of soil salinity over time. its influence on capillary rise; the shallower the depth, the The current study was designed to assess the effect of higher the contribution of groundwater to salinization. water table depth and its salinity on salinization of soil Generally, the decreasing trend of the maximum capillary from evaporation (uncropped soil as follow soil) of saline rise in various uniform soils under a given evaporative solutions. This research conducted in field condition in a demand follows the descending order as sandy loam, semi-arid region- Ahvaz, Iran, with simulating by loam, clay loam, clay medium and coarse sand. Hadas lysimeters. and Hillel (1968) and Ashraf (2000) measured the rate of evaporation as a function of the water table depth and the evaporability. The air circulation over the top of the soil MATERIALS AND METHODS columns was varied to obtain different evaporability The investigation comprised three separate experiments with 42 condition. The experiment was conducted in constant lysimeters. Lysimeters contained (from the base of the soil temperature and no radiation. In addition, evaporation columns) thickness of 5 cm gravel and 5 cm of sand to allow was measured for different water table depth under unrestricted exchange with the supply of groundwater, then conditions of increasing evaporability. His experiments contained soil to top of lysimeters. Air-dry topsoil of the soil from research field of Chamran University of Ahvaz was sieved through a shown that for the loess-derived soil, an inverse relation 2 mm mesh. The soil was packed as uniformly as possible in 10-cm exists between the evaporation and evaporability. He layers to bulk density of 1.35 g/cm3. Selected properties of the soils discarded the salt effect as an explanation and promoted are given in Table 1. The field was excavated in order to place the the idea that the gradual drying of the soil surface zone in lysimeters at a certain depth so that their top was on level with the effect created a two-layer condition. Water movement ground surface. The excavated soil was then placed back into the across the dry surface layer would then be in the vapor lysimeters and the surrounding space. The experiments with saline (3 g NaCl/L + 3.5 g CaCl2/L) and three water table depths (300, 500 phase only. Prathapar and Qureshi (1999) and Ali et al. and 800 mm) (Rose et al., 2005). This study was conducted under (2000) investigated and discussed in detail the effects of field conditions. The soil columns were cylindrical PVC tubes of 120 soil type, ground water quality and ground water depth on mm inside diameter. The base of each column was closed by a salinisation of soils, as well as the other researchers plastic disc. To saturate the soil, tape water was introduced from showed that the high soil salinity and alkalinity restricts the bottom to avoid trapping air. Water was then allowed to drain while keeping the soil surface covered and allowed to stand for one crop growth by reducing the osmotic potential, day in order to deplete the detention storages by gravitational force. decreasing nutrient availability and soil physical quality The soil columns were weighed and then placed in excavated parameters (Gokalp et al., 2010). Also, Many researchers places and left for evaporation in field condition. The Marriot tank investigated experimentally the movement of water and systems were installed to conduct constant water table depth in salts above saline water tables in constant evaporability experimental period. The lysimeters were weighed and sectioned condition (Hassan and Ghaibeh, 1977; Chen, 1992; (10 cm layers) after 5, 15, 30, 45, 60 days and at the end of experiment to determine gravimetric water content. Electrical Shimojima et al., 1996; Ali et al., 2000; Rose et al., 2005; conductivity (ECe) was determined in the end of research period. Gowing et al., 2005). Further experimental and The experiments started on April, 10th, 2010. The rate of capillary theoretical work is needed on the upward movement of rise was taken as the volume of water supplied by the Mariotte 6070 Sci. Res. Essays

Figure 1. Pan evaporation data.

Figure 2. Average evaporation.

tank. This volume was monitored periodically. Potential evaporation evaporation was measured using pan evaporation. The was monitored with a Class-A evaporation pan that located at 10 m amount of EP was equal about 8 mm/day on the first day from the lysimeters. and was 14 mm/day in the end of experiment. Moreover,

Figure 2 shows the average evaporation from the soil RESULTS surface versus pan evaporation data. After 10 days, in treatment D300 (D = 300 mm), the average evaporation In a non-steady-state, the rate of evaporation equals the from soil surface decreases from 7 mm/day to 5 mm/day sum of the rates of water loss from the Mariotte tank and and its value was about 4 mm/day after 20 days. In these the water depleted from the soil profile over a given time columns, the evaporation rate was about 3 mm/day at the period. Figure 1 shows the daily evaporation from pan end of the experiment. In the D500 (D = 500 mm), the evaporation, EP. Under field conditions, daily potential rate of evaporation decreased to about 3.5 mm/day within Jalili et al. 6071

Figure 3. Comparison between evaporation of three water table depths.

Figure 4. Salt profile in the end of experiment.

10 days, and to 2.2 mm/day within 20 days and followed from soil surface) are 293.2, 185.5 and 95.3 mm, in by a slower continuous drop to 1.4 mm/day within 80 treatments D300, 500 and 800, respectively. Figure 4 days. In treatment D800 (D = 800 mm), the evaporation explained the profiles of salt content at end of rate of about 7 mm/day decreased rapidly to about 2.6 experiment. The ECe of the soil increased with mm/day, within 10 days. It then fell to 1.2 mm/day during evaporation at soil surface. As shown in Figure 4, the salt the following 20 days and finally decreased slowly to a concentration increased with high slope between 200 and rate of 0.2 mm/day. Amounts of accumulated evaporation 10 mm in D300 treatment and the concentration from soil surface were presented in Figure 3. Amount of remained at or closes to the 10.55 dS/m below the depth cumulative evaporation (E-accumulated, E is evaporation of 200 mm. In the treatment of D500, the salt 6072 Sci. Res. Essays

Figure 5. Water content profile on the first day.

Figure 6. Water content profile in the end of experiment.

concentration profile increased from 9.57 to 56.88dS/m treatment D800. Below the depth of 200 mm, the between 200 and 10 mm and in the end of experiment. concentration did not change noticeably and remained This values rose more slightly than that D300 column. about 10.55 dS/m. Below 200 mm, the concentration of salt changed from Figures 5 and 6 shows profiles of volumetric water 9.55 to 10.55 dS/m. The salt concentration between 200 content at start and end of the experiment. The upper and 10 mm increased with low slope and increased from part of the soil profile dried quickly because the 9.95 to 24.01dS/m below the depth of 200 mm in evaporative demand was greater than the ability of the Jalili et al. 6073

soil to conduct water from the water tables. in a moist soil surface and higher evaporation and consequently more salt accumulation in soil surface. The peak salt concentration was obtained below the depth of DISCUSSION 10 mm and might have been caused partly by the formation of a crust at the soil surface and partly by the Objective of this investigation were to observe what effect mixed vapour-liquid flow (Asghar 1996; Gowing and the depth of shallow saline water table have on soil Asghar, 1996; Gowing et al., 2006). As soon as salinity and their influence on salt distribution in the soil evaporation started, the water content of the top layer profile. If the surface of the soil is wet, the evaporation began to decrease due to the greater evaporative rate stays nearly constant for some time and then demand than the ability of soil to conduct water and suddenly decreases. When the soil dries so sufficiently vapor became a large proportion of the total water flux in that water cannot be supplied to the surface fast enough this zone. Salt therefore started to accumulate below this to meet the evaporative demand, the soil surface dries layer. A crust then formed at the surface which kept the and the evaporation rate is reduced (Mehmet et al., underlying soil wetter for a longer period of time and 2005). On the other hand, reduction of evaporation rate allowed more salt to accumulate below the crust. from soil surface in all treatments over the time might be Consequently, a peak concentration of salt occurred in result of reduction of the hydraulic conductivity (Konukcu, this layer. The profiles of salt accumulation were 1997; Rose et al., 2005). Evaporation reduction trends; consistent with the water-content profiles in that salt was Figure 2 showed similar patterns in all treatments. Figure deposited mainly below the evaporation front (Figures 5 2 shows that under the same atmospheric demand, and 6). When the depth of the evaporation front became evaporation from the soil surface was significantly static, salt accumulated only in this constant depth affected by the depth of the water table. In addition to (evaporation front) (Rose et al., 2005). Note that the result of this study, it explained that water table depth increase in the salt concentration in top layer of columns affect the cumulative evaporation rate. The results is not only due to the upward movement of saline water presented here are comparable with those found by from the water table, but also to molecular diffusion Gowing et al. (2005) and Rose et al. (2005). Results of downward in response to the concentration gradient. this study showed that cumulative amount of evaporation Analysis of data indicates that the depth of water table is in small columns (D300) is largest value (Figure 3). This significant effect (5% significance level) in evaporation can be explained by the replenishment from the capillary and soil salinity. fringe was sufficiently quick to prevent water contents Moreover, the change in the water content to a decreased and the ground water can supply the restricted value indicates the transition region (Menenti, evaporation demand better than D500 and 800 1984; Bastiaanssen et al., 1989; Shimojima et al., 1996), treatments. It can be seen that the water contents in the because, when the phase of flow changes from liquid to soil profiles decreased towards the soil surface but the vapour, hydraulic continuity is broken. Consequently, magnitude of the change were much greater in the upper water content decreases sharply (Rose et al., 2005). layer than closer to the water table. When the water table Further evidence is obtained from the characteristic depth was increased, the upper soil profile dried more shape of the salt profile and the interrelation between salt quickly because the capillary rise from the water table movement and water content (Jackson et al., 1973; was slower. The ratio of actual evaporation from the soil Nakayama et al., 1973; Shimojima et al., 1996). Salts in treatment D300 was high on the first 2 weeks. move only with the liquid water and are deposited when However, the reduction in capillary rise during the the phase changes from liquid to vapour. Therefore, the following weeks was due to a decrease in hydraulic maximum deposition in the soil profile is expected where conductivity result in reduction of evaporation. In other the phase transformation occurs. This zone is located treatments, D500 and 800 capillary rise was high during between those dominated by either liquid or vapour flow. the first week and more reduction in capillary rise carried The results presented here are comparable with those out due to increase in water table depth. This is due to found by Shimojima et al. (1996) and Rose et al. (2005). when the water table depth increased; the upper soil profile dried more quickly because of the contribution from the water table was slower. As be shown, the lower Conclusion the lysimeter had most salt accumulation in top layers, because of that had most evaporation rate from soil In this investigation, we conducted experiments on a clay surface. loam soil, with shallow saline water tables under natural The deeper soil column resulted in a dry soil surface evaporative demand in Ahvaz, monitoring amount of that reduced the amount of evaporation. Water evaporation from bare soil, and observing profiles of soil movement required a combination of capillarity and vapor water and salinity over periods of up to 80 days. The diffusion to reach the soil surface from the bottom of the results showed that salts mainly accumulated in the soil soil columns. The shallower depth to water table resulted surface and transition zone. Furthermore, result showed 6074 Sci. Res. Essays

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