بررسي آزمايشگاهي تغييرات زماني شوري زه‌آب كشاورزي مربوط به عمق‌هاي مختلف زهكش

Experimental study of temporal variations in the salinity of agricultural drain water at different drain depth

وجود آب زيرزميني كم‌عمق با شوري بالا در مناطقي مانند خوزستان سبب خارج شدن زه‌آب با كيفيت پايين از زهكش‌ها مي‌شود. يكي از عوامل موثر در كيفيت زه‌آب خروجي در اين مناطق عمق نصب زهكش است كه سبب خروج نمك‌هاي آب زيرزميني در اثر جريان‌هاي شعاعي مي‌شود. هدف از انجام اين پژوهش، تعيين روند تغييرات شوري زه‌آب خروجي و آب زيرزميني نسبت به زمان، در عمق‌هاي مختلف استقرار لوله‌هاي زهكش است. براي انجام اين پژوهش، از يك مدل آزمايشگاهي با ابعاد 8/1 در 1 در 2/1 متر استفاده شد. زهكش‌ها در عمق‌هاي 20 ،40 و 60 سانتي‌متري نصب شدند. شوري آب زيرزميني حدود 65 دسي‌زيمنس بر متر تنظيم شد و آبياري به وسيله آب با شوري 321/0 دسي‌زيمنس بر متر و دبي‌هاي 14/0، 11/0 و 07/0 ليتر بر ثانيه انجام شد. نتايج اين پژوهش نشان داد كه با افزايش عمق و دبي هر زهكش مقدار نمك خروجي نيز افزايش مي‌‌يابد به طوري كه بيشترين مقدار براي عمق نصب 60 سانتي‌متر و دبي 14/0 ليتر بر ثانيه با مقدار 3/9 كيلوگرم و هدايت ‌الكتريكي 65/39 دسي‌زيمنس بر متر و كمترين مقدار براي عمق نصب 20 سانتي‌متر و دبي 07/0 ليتر بر ثانيه با مقدار 01/3 كيلوگرم و هدايت ‌الكتريكي 37/29 دسي‌زيمنس بر متر است.

Increasing human population and reduction in land available for cultivation are two threats to agricultural sustainability. So, the biggest issue globally for agriculture is the need to produce enough to feed the growing world population. Another challenge is soil salinity. Soil salinity is an enormous problem for agriculture under irrigation. Millions of hectares of land throughout the world are too saline to produce economic crop yields, and more land becomes nonproductive each year because of salt accumulation. Irrigation in these areas causes the salts leached from the soils to build up in groundwater. Eventually, the groundwater table rises bringing saline water to the surface and making soils unsuitable for cultivation. Lack of proper drainage system leads to salinity in the soil and salts tend to accumulate in the upper soil profile. According to experimental and field evidence, subsurface drainage is the essential intervention to maintain a suitable growing environment for crops. According to previous studies the design criteria of the drainage system are too conservative and can be modified. So the main objective of this study is to use the experimental model for designing drain depth that can maintain desired salt concentration at the base of the root zone with less discharge water as compared with conventional drainage design equation. Experiments were performed in a box with 1.8 m length, 1m width and 1.2 height. Drain depths were 20, 40 and 60 cm below the surface and the spacing was 180 cm. A separator layer (filter layer) was used on the surface of the drain tube to prevent fine soil particles from leaving the box into the drain tube. The impervious bottom layer was designed in the model. The surface-irrigation system was designed for irrigation from the top of the soil box. Saline groundwater prepared by mixing NaCL salt with fresh water. The electrical conductivity of groundwater was around 65 dS/m and the irrigation water salinity was around 0.321 dS/m. Irrigation flow rates were 0.14, 0.11 and 0.07 l/s. The observation piezometers were installed between lateral drains at different depths and with spacing of 15 cm from each other to monitor the Spatio- temporal variations of groundwater salinity. Groundwater samples were collected from piezometers that installed on different distances and depth and the samples of drain outflow were collected with time intervals of 5 to 10 minutes for electrical conductivity analyses. The electrical conductivity measured by the Senso Direct Con 200. The results showed that there was a hydraulic gradient around the drains decreasing towards the midpoint. As expected, there was a reduction of electrical conductivity in time. The results also showed that the irrigation water mixed with the groundwater and was diluted because the salt was removed by drain tubes. So, the electrical conductivity was lower near the drain tubes and highest at the bottom of the box, as this effect was apparent during the experiment. The rapid decreasing of groundwater electrical conductivity was due to less saline irrigation water that entered the saline groundwater enhanced salt to the drain tubes. However, the results convincingly demonstrate significant differences in the rate of salt fall at different irrigation flow rates. The results showed that the high irrigation flow rate has greater potential for salt removal from groundwater during the experiments. In addition, drain depth affected the water table depth and had more effect on the salinity of the drain water. The electrical conductivity of drainage water for drain depth of 60 cm and irrigation water of 0.14 l/s was 39.65 dS/m. However, the electrical conductivity of drainage water for drain depth of 20 cm and irrigation water of 0.07 l/s was 29.37 dS/m. The results demonstrate that the drain water concentrations decreased exponentially with time for all drain depths. At the end of the experiments, drain water concentrations were almost the same for all drain depths.