The effects of shallow saline groundwater on evaporation, soil moisture, and temperature distribution in the presence of straw mulch

Mitigating evaporative water loss from terrestrial surfaces is of central importance to water resources management in arid and semi-arid regions. This study was intended to experimentally address the effect of straw mulch layer on soil evaporation and temperature distribution in the presence of shallow saline groundwater. A factorial-based experiment with a completely randomized design was carried out in mini-lysimeters (MLs) with different concentrations of saline groundwater and soil types, with and without straw mulch. The lysimeters were placed on the soil surface in the field. Water table in MLs was kept at the depth of 60 cm, and evaporation rate, soil moisture content, soil salinity, and temperature were continuously monitored. The analysis of variance (ANOVA) indicated significant differences in the soil evaporation rates due to the effects of soil types (i.e., loam and sand) and straw mulch (p< 0.01). The results showed that soil temperature fluctuations at the 5 cm depth in loamy soil with and without mulch were 11.5 and 17.5 C, while in sandy soil the fluctuations rates were 15 and 18.5 C, respectively. The application of a mulch layer was found to significantly reduce the evaporative loss by 27 and 8% in loamy and sandy soils, respectively.


INTRODUCTION
Soil evaporation as one of the key processes in the hydrological cycle and water balance of arid and semi-arid regions (Shaw et al. ; Harwell ; Assouline et al. ) can be divided into three stages. During the first stage, a rather constant evaporation rate controlled by atmospheric forcing is kept for a significant time (Shokri et al. ). The surface of the soil, then, begins to dry and a sharp drop is observed in the evaporation rate during the so-called falling stage (van Brakel ; Prat ; Shokri & Salvucci ).
Eventually, at the onset of the third stage, the vaporization plane recedes into the soil profile and the diffusion processes govern vapor transfer to the soil surface (Aminzadeh & Or ).
Soil evaporation is primarily affected by the intrinsic soil properties (e.g., soil texture, hydraulic properties), the initial water content and the depth of water table, as well as the atmospheric forcing (e.g., solar energy, air temperature, humidity, and wind speed) (Rasheed et  Soil texture is, indeed, one of the important parameters in water retention and evaporation from the soil, and finetextured soils tend to retain more water as compared with the coarse-textured ones. As such, the evaporation rate in fine-textured soils is higher than that of the coarse-textured ones. On the other hand, in fine-textured soil when groundwater is shallow, a significant amount of water evaporates through the capillary rise (Trenberth et  Hayashi ). For instance, in arid regions with high potential evaporation, about 50-70% of the rainfall (needed for plants' growth) is lost via evaporation (Jalota & Prihar ). Considering the complexity of plant transpiration, various methods such as plowing or mulching have been invoked to control the evaporative soil water loss (Freitas et al. ). Mulching indeed covers soil surface by adding organic and inorganic materials to it so as to increase the productivity of water, and provide a more favorable environment for plant growth (Parmanik et al. ). Vial et al. () reported that the addition of straw mulch reduced evapotranspiration about 114-161 mm, which, in turn, increased water productivity on corn farms. Similarly, Chen et al.
() concluded that the application of mulch could effectively keep soil moisture and reduce evapotranspiration.
Investigating the effect of wheat residues on soil surface evaporation, Bhatt & Sanjay () showed that the presence of mulch suppressed the evaporative loss up to 80%.
Likewise, Akhtar et al. (), in a study conducted in Northwest China, reported that straw mulch increased soil moisture by 4.7%, leading to a 3% temperature decrease in soybean farms. The results of Yan et al. () also demonstrated that the presence of straw mulch enhanced water use efficiency of wheat farms enabling dry land farming.
According to McMillen (), a layer of wheat straw mulch, grass, and fresh leaves with a thickness of 5-10 cm could increase moisture content up to 10% compared with bare soil. However, the results of Ashrafuzzaman et al. () were in disagreement with the results of other studies as they did not find significant differences in retaining soil moisture content among different types of mulch covers.
In turn, soil temperature, as one of the most important factors affecting heat and mass exchanges in soil, plays a significant role in various processes ranging from seed germination, plant growth, root development, soil microorganism's activity, transport and dissolution of pollutants and herbicides, as well as soil salinity (Grifoll et al. ) to soil ventilation, infiltration, and evapotranspiration (Nassar & Horton ). In some studies, it has been acknowledged that the application of straw mulch increases soil temperature (Nath & Sarma ; Ramakrishna et al. ). By contrast, the results of other studies have shown that mulch reduces soil temperature (Wu et al. ).
Similarly, soil temperature distribution, water content in soil profile and salt precipitation induced by evaporation in arid and semi-arid regions have been extensively studied (Jalili et al. ; Ojo et al. ; Mengistu et al. ).
However, some evidence shows that the existence of shallow saline water tables with high potential evaporation in these regions (Gholami et al. ) results in salt accumulation at soil surface (along with its associated problems in soil quality). This, thus, highlights the need for using efficient measures to mitigate the evaporative losses in soil moisture by considering the concurrent impacts on key thermal and transport processes of soil. Accordingly, this study was intended to systematically investigate the effect of straw mulch on soil evaporation, soil moisture, and temperature distribution, as well as salt precipitation in a set of lysimetric experiments in the southwest of Iran (Khuzestan province) with a shallow saline water table and high atmospheric evaporative demand. Farmers in Khuzestan province struggle with shallow and saline groundwater that results in considerable evaporative water loss and salt precipitation on soil surface. Indeed, the main objective of this study was to simultaneously evaluate the effect of using mulch as a protective layer on the evaporation rate, water content, salinity, and temperature distribution in soil profile with different textures, and also provide a scientific basis for the application of such a simple and cost-effective technique to control the problems associated with evaporative soil water loss.

Experimental site
The experiment was conducted during the summer period with the maximum air temperature and potential evaporation According to Ambereje method, the climate of this region is semi-desert with hot and long summers and mild and short winters. Table 1 shows the meteorological parameters including the minimum and maximum air temperature, wind speed, sun hours as well as the minimum and maximum relative humidity in the study period.

Preparation and management of mini-lysimeters (MLs)
In the experiment, cylindrical PVC tubes with an inner diameter of 25 cm and length of 75 cm were used as MLs to investigate the effect of mulch cover on soil evaporation in a controlled domain. To that end, the lysimeters were placed on soil surface in the field and insulated with 4 cmthick glass wool sheets to minimize the lateral heat fluxes.
Using a Mariotte bottle system (Rose et al. ), during the experiment, the same as the site where the lysimeters were placed, the water table in mini-lysimeters (MLs) was kept at a depth of 60 cm (Karimi & Naseri ) to simulate the effect of the actual water table on evaporation, soil moisture, and temperature distribution ( Figure 1).
Indeed, each lysimeter was initially saturated, and after the gravitational drainage in which soil water content was approximately equal to field capacity the experiment began.
The salinity of irrigation water in the study region varies in the range of 1.5-2.5 dS m À1 during the year. To prepare the saline water with electrical conductivity (EC) of 5, 10, and 15 dS m À1 , we used 1.5 g/L NaCl þ 2 g/L CaCl 2 , 3.5 g/L Nacl þ 4 g/L CaCl 2 , and 7 g/L NaCl þ 10.5 g/L CaCl 2 , respectively (Rose et al. ). Then, two types of soil with different physical characteristics (as shown in Table 2) along with two soil cover conditions (i.e., M: mulch and nM: no mulch) were used. Furthermore, wheat straw (0.35 kg m À2 ) was applied as mulch treatment in this study.
Measurement of soil moisture, salinity, and temperature distribution The volumetric water content, soil salinity, and soil temperature in MLs were measured at five depths, namely, 5, 10, 20, 30, and 50 cm. Then, the digital temperature sensors (DS18b20, China) connected to a data logger were used to record soil temperature hourly, while soil moisture and EC were monitored on a daily basis using time domain reflectometry (TDR) probes with 0.01% accuracy (HD2, IMKO, Germany).

Sensor calibration
Considering different soil textures in the experiments, soil moisture and temperature sensors were calibrated. The HD2 moisture probes were then placed inside separate soil pots to measure daily soil moisture concurrently by weighing the pots. Finally, the best fitting curve (not shown here) was obtained using the data recorded by HD2 probe and the volumetric soil moisture content measurements.

Soil water retention curves (SWRC)
A sandbox and pressure plate apparatus were used to obtain water retention curves of the two soil types. The van Genuchten () model was then fitted on the measured data to determine the corresponding parameters (Equation (1), Figure 2): where θ r and θ s are the residual and saturated moisture content, respectively, α is related to the inverse of the air entry suction (cm À1 ), n and m are the shape parameters in which m ¼ 1 À 1/n, and h is the matric potential (cm).
The maximum height of capillary rise (h max ) within the soil columns can be calculated using the shape parameters of soil moisture curve (α and n) based on Equation (2) Figure 1 | Experimental setup for studying evaporation, soil temperature distribution and water content from different soil types in the presence of shallow water table.

Soil water evaporation
The simplest form of soil water balance indicates that in a given volume of soil, the difference between the amount of input and output water shows soil moisture variation (Hillel ): where P is the precipitation, I is the irrigation, G o stands for the contribution of groundwater in evaporation (here, water consumption from the Mariotte bottle), R o is the runoff, E s and T r are the water loss via evaporation and transpiration, respectively, and finally, ΔW shows the variation of soil water content. Considering that our measurements were conducted in the lysimeters under controlled conditions with no irrigation, therefore I ¼ 0. During the experiment, there was no rain (P ¼ 0), and thus no run-off (R o ¼ 0). In addition, there was no plant in this experiment, therefore T r ¼ 0. By simplifying Equation (2), the evaporation rate from the soil surface is obtained as: On the other hand, the maximum soil evaporation in the presence of shallow groundwater (e max ) depends on the water table depth and soil characteristics (β and Ks) where Ks is the saturated soil hydraulic conductivity, L is the water table depth, and β is an empirical factor (Gardner

).
Daily potential evaporation data were obtained from a Class-A evaporation pan in a meteorological station located 5 km away from the experimental site.

Statistical analysis
The study was conducted as a factorial-based experiment using a completely randomized design with three replica-

Variation of soil water content
Daily variations of moisture content in two soil textures (i.e., loamy and sandy soils) with and without straw mulch in a three-month period are depicted in Figure 3. As seen, in all treatments the soil moisture content in the surface layers was lower than that in the deeper layers.

Soil salinity
The vertical variations of salt content in the soil profiles for all treatments on the 5th, 45th, and 90th day of the    Table 3. The finding revealed that the single effect of groundwater salinity, soil type, and mulch on evaporation was statistically significant (p < 1%). However, the interactions between soil texture and groundwater salinity, and mulch cover and groundwater salinity on evaporation were not significant. Interestingly, the interaction between soil texture and mulch on evaporation was notable (p < 1%). In addition, the results of mean comparison indicated that the evaporation rate in loamy soil was 17% higher than that of sandy soil (Figure 8(a)), and straw mulch cover reduced the evaporation rate by 26% compared to that in the bare soil (Figure 8(b)). Among different salinity levels of groundwater, S1 treatment had more soil evaporation than S2 and S3 (Figure 8(c)). Besides, the highest amount of evaporation was from bare soil with the groundwater salinity of 5 dS m À1 while the lowest rate of evaporation was from the mulch-covered soil with salinity of 15 dS m À1 . Also, in S1 treatment, the mulch cover during the experiment period caused a 20% reduction in the evaporation rate as compared to that in the bare soil ( Figure 8(d)).
By measuring the water level in Mariotte bottles and calculating the water balance in the soil column, the cumulative evaporation for loamy soil in the three salinities of S1, S2, and S3 with and without mulch cover was obtained as 246, 161, 106 and 338, 217, 150 mm, respectively. These results indicate that with increasing salinity of groundwater and its potential effect on water vapor pressure, the cumulative evaporation decreases (Bittelli et al. ). Also, in accordance with Equation (5), the maximum amount of evaporation in sandy soil was found to be 12.8 mm day À1 while it was 22.4 mm day À1 in loamy soil. Figure 9 shows the relation between the cumulative evaporation (ΣEs) and the total potential evaporation (ΣPE). As seen, there was a linear relationship between the cumulative evaporation and the potential evaporation in all treatments. In bare soils, the ratio of the cumulative evaporation to the total potential evaporation (ΣEs/ΣPE) was much higher than that of the mulch-covered soil. For salinity of S1, the ratio for loamy and sandy soils without coverage was about 0.78 and 0.48, respectively, while in the presence of mulch it was found to be 0.54 and 0.43. The values of water losses for different treatments in both soils relative to the potential evaporation are shown in Table 4 to highlight the relative contribution of each treatment in the soil evaporation. As expected, the results revealed that in all the groundwater salinity treatments and both soil textures, the cumulative evaporations in bare soil were more than those of the mulch-covered soils. Moreover, loamy and sandy bare soils in the S1 treatment had the highest ratio of cumulative evaporation to the total potential evaporation with 55.9 and 43%, respectively. However, both loamy and sandy mulch-covered soils in the S3 treatment had the lowest ratio with 24 and 16%, respectively. The results of Table 4    groundwater, as it was in the present study. Indeed, by controlling the evaporation rate, mulch has desirable effects on the moisture regime of the soil. As expected, the moisture content in soil treatments with mulch for both sandy and loamy soils was found to be higher than that in the bare (uncovered) soil during the experiment ( Figure 3). The moisture retention in loamy soil was, in turn, much higher than that of the sandy soil. This could be attributed to the higher clay content in loamy soils than that of the sandy soils. Loamy soil with smaller particles (more silt and clay content) has a larger surface area and much smaller pore size distribution which allows it to hold more moisture. Due to proximity to the water table, moisture variations in the lower layers (i.e., 20 cm depth and lower) were slight in both treatments until the end of the experiment period. In addition, there was no significant change in soil moisture content as it approximately remained at the same extent as it was at the beginning of the experiment. Similar to our findings, Figure 9 | Comparison between the cumulative potential evaporation and the cumulative actual evaporation for different treatments: M and nM stand for mulch and no-mulch; S1, S2, and S3 are salinity levels of groundwater with ECs of 5, 10, and 15 dS m À1 .

Salt accumulation in soil profile
In arid lands with saline groundwater, salinity control in the root zone is considered as one of the most important factors in seed emergence and stand (Meiri & Plaut ; Dong et al. ). The results of this study clearly showed that soil evaporation in the presence of shallow saline water

Evaporation from shallow groundwater
In the early days of the experiment, due to the high moisture content of the samples, the evaporation rate was high and the role of water table on the evaporation from the soil surface was negligible. As such, there was sufficient moisture for evaporation in the soil profile. With the reduction of soil moisture, the suction gradient and thus the capillary rise from the water table to the soil surface was enhanced. Therefore, climatic conditions such as air temperature, relative humidity, and radiation determined the evaporation rate.
As shown in Figure 7, the evaporation pattern in the three salinity treatments of S1, S2, and S3 was almost the same following the atmospheric forcing. Also, with increasing the salinity of groundwater, the evaporation rate reduced in all treatments. Nachshon et al. () indicated that one of the main mechanisms affecting evaporation under saline conditions is the accumulation of salt crystals in the soil matrix reducing the permeability of the soil profile. By comparing the evaporation rates in sandy and loamy soils without mulch (Figure 7), it can be seen that sandy soils are more rapidly dried, and thus the amount of evaporation is less relative to loamy soils. It was also observed that the evaporation from the coarse-textured soil (sand) was more sensitive to the water content and water table level changes in comparison with that in the fine-textured soil (loam).
Moreover, several other studies have reported that the upward flow due to the capillary rise in the fine-textured soils continues for longer periods before evaporating pores completely dry (Nassar & Horton ). In effect, as surface pores dry out, water loss occurs with diffusion processes within the soil medium, as the vapor diffusion requires more energy than the capillary rise (Noy-Meir ; Van de Grend & Owe ). Moreover, by increasing the distance from the water table, the amount of water content then decreases gradually and the upper limit in capillary rise becomes insignificant (especially in sandy soil where groundwater does not reach the soil surface). As such, soil is divided into two wet and dry regions below and above the vaporization plane, respectively. The distance of water

CONCLUSIONS
This study investigated the effect of shallow groundwater with different salinity levels on the surface evaporation, soil salinity, water content, and temperature distribution in the soil profile. It also addressed the impact of mulch cover on these parameters. Regardless of the soil type, it was found that mulching practice can effectively reduce soil temperature fluctuations and increase the water content in the surface layer (0-5 cm). The results also showed that with increasing the salinity of groundwater, due to the increase in salt concentration, the evaporation rate in all the investigated treatments decreased. Moreover, straw mulch cover was found to reduce the evaporation rate by 26% compared to that in the bare soil. Also, the findings highlight the importance of groundwater salinity in changing the soil quality as well as the positive impacts of mulch cover on evaporation suppression and also the improvement of water storage in arid and semi-arid regions.
It is recommended that other researchers conduct further studies, especially on the contribution of shallow groundwater with different salinity levels to the water requirement of plants in the presence of mulch coverage.
Furthermore, it is suggested that mulch covering with different thicknesses be used in other studies to couple the transfer of water and heat in the soil profile.