اندازه‌گیری مستقیم و شبیه‌سازی اثر میزان سیلاب بر نرخ تغذیه به آبخوان در سامانة پخش سیلاب گربایگان مقالة

ارزیابی نرخ واقعی تغذیه به سفره بر اثر روش‌های مختلف استحصال سیل از موارد مهم پژوهشی است. برای آگاهی از میزان تغذیه مصنوعی آب زیرزمینی بر اثر عملیات آبخوانداری سه چاه با عمق تقریبی 30 متر در یکی از نوارهای سامانه پخش سیلاب گربایگان، با قدمت 39 سال، به‌ترتیب در کاربری‌های جنگل دست‌کاشت آکاسیا، زمین بایر، و پوشش مرتعی حفر شد. ویژگی‌های هیدرولیکی لایه غیراشباع خاک در تمام نیمرخ آبخوان به‌صورت میدانی و آزمایشگاهی اندازه‌گیری شد. یکی از چاه‌ها‌ با حس‌گرهای TDR که از پیش برای خاک محل پژوهش واسنجی شده بود تجهیز شد. حس‌گرها در حفره‌هایی که در دیواره چاه (از پیش عایق‌بندی شده) آماده شده بود از سطح تا عمق 30 متر کارگذاری کابل‌های آن‌ها تا بالای چاه کشیده شد. مقدار حجم آب خاک از مرداد 1389 تا خرداد 1399 به‌طور پیوسته یک نوبت در هفته در زمان‌های فاقد رخداد سیل و بارش و یک نوبت در روز در هنگام بعد از رخدادهای سیل و بارش اندازه‌گیری شد. داده‌های بارش و ارتفاع آبگیری نوار تغذیه در کنار چاه مشاهده‌ای یادداشت شد. تغذیه در محیط غیراشباع با روش بیلان آب خاک با کمک داده‌های اندازه‌گیری آب خاک در لایه‌ها ارزیابی شد. مقدار تغذیه با راه‌حل معکوس به‌وسیله مدل‌ Hydrus یک بُعدی شبیه‌سازی شد. نتایج نشان داد که جبهه رطوبتی پس از وقوع سیل در عمق چهار متر جایی‌که یک لایه خاک ریزبافت روی لایه درشت بافت سنگ‌ریزه‌دار قرار دارد، مختل شده است. تفاوت زیاد در میزان هدایت هیدرولیکی دو لایه متوالی سبب تبدیل حرکت جبهه‌ای به حرکت انگشتی در این عمق شده است. اعتبارسنجی نتایج شبیه‌سازی جریان با مدل واسنجی شده Hydrus در برابر مقادیر مشاهده‌ای (با RMSE، 3.45 سانتی‌متر و R2، 0.994) نشان داد مدل با موفقیت قادر به تخمین جریان آب پس از وقوع سیل بوده و آماره‌های صحت‌سنجی نتایج نزدیکی به واقعیت ارائه کرده است. نسبت تغذیه بین 26-84 درصد با میانگین 55 درصد برای کل وقایع و 84 - 63 درصد با میانگین 75 درصد برای سیل‌های بزرگ (با ارتفاع ماندابی بیش از 50 سانتی‌متر و در طول بیش از 24 ساعت) بر اثر سیل و بارش را می‌توان یکی از دست‌آوردهای پژوهش حاضر دانست.

Introduction Water scarcity due to climate change and growing water demand in different consumption sectors is a major environmental crisis that drives arable lands to a state of degradation, especially in dry regions. Artificial recharge of groundwater (ARG) through floodwater spreading (FWS) which is a potential measure for reversing this emerging trend is investigated in this research. Floodwater harvesting has become an increasingly important technique to improve water security and caused a renewed interest in research and implementation. According to the diverse objectives and methods of implementing artificial recharge of groundwater (ARG) systems, various factors must be considered when choosing a method for quantifying recharge. Therefore, the rate of aquifer recharge is one of the most difficult items to measure in groundwater (GW) resources evaluation. In the soil water balance method (and in the Zero Flux Plane method), soil water movement is inferred by measuring the changes in water content of the soil profile by gravimetric sampling or automatic devices. These methods have not been proven satisfactory in low flow conditions, as there is often insufficient resolution to detect the movement of small quantities of water. Therefore, other methods, based on hydraulic conductivity, potential gradients, and directly calculated water fluxes for unsaturated flow were developed. The Buckingham-Darcy law can be used under the steady flow condition where water contents and fluxes change with depth but do not vary as a function of time. It has been employed in arid and semiarid conditions for recharge estimation. or for assessing the exchange flow between the surface water reservoir and GW. The method requires measurements or estimates of the vertical total head gradient and the unsaturated hydraulic conductivity at the ambient soil water content following the Buckingham-Darcy equation. The overall objective is to evaluate a floodwater spreading system that was installed in 1981 at the Gareh Bygone Plain, southern Iran for recharging the groundwater table. Materials and Methods To assess the artificial recharge of groundwater through turbid floodwater spreading, three wells, ~30 m deep, were dug in a 37-year-old recharge basin in planted Acacia forest, bare soil, and pasture land uses, respectively. Soil hydraulic parameters of the vadose zone layers (30 m thickness) were measured in the field and laboratory. One well was equipped with pre-calibrated TDR sensors throughout the well profile for measuring the changes in soil water content along the vadose zone. The volumetric soil-water content was measured continuously from Sep. 2010 to Sep. 2020 with closer temporal increments after floodwater spreading events. Rainfall, ponding water depth, and duration were also measured. Recharge through the vadose zone was assessed by the soil-water balance (SWB) method, as measured in the field as well as by calibrating the Hydrus-1D (H1D) model through the inverse solution. Results and discussion Results showed that the wetting front was interrupted at a layer with fine soil accumulation over a coarse-textured gravely layer at a depth of ~4 m. The large differences in hydraulic conductivity of the two successive layers seemed to cause the transformation of the wetting front water movement into fingering flow. The changes in downward water flux complicated TDR measurement after the depth of 4 m. However, noticeable but temporary changes in the soil water content were detected in some of the layers below the 4 m was evidence for fingering flow after the flood events. Validation of the simulated flow by the H1D model vs. the one observed by SWB (with RMSE 3.45; R2 0.994) showed that the model performed well in flux estimation. The recharge ratio was calculated for the 2010 to 2020 events as 26 to 84 average of 55 % for all events and 63 to 84 average of 75 % for large impounded floodwater in the basin, respectively. Conclusion Although a reliable set of data is obtained for calculating recharge at the very location of this study, up-scaling of the results for the entire floodwater systems and for the other flooding events with extreme volume and flow rate needs an extended investigation period and thorough identification of the underlying layers. The determined hydraulic properties of the RLs obtained in this study will be utilized in future research works in the FWS systems in our study site. The contributions of this thesis can be summarized as a) development of approaches for application, calibration, and validation of existing models with limited available data, b) the incorporation of new concepts into the models used, c) generating a unique and robust field data set to support the modeling approaches and d) provision of new information in the context of floodwater harvesting and its impact on groundwater recharge. Floodwater harvesting, especially in the form of FWS, is an emerging issue in water management in dry regions, which needs a better understanding and evaluation of its impact on the surrounding environment. Small-scale but nature-friendly water management plans, such as FWS systems, are seriously criticized since there are numerous methods, which are more attractive in terms of investments and money return to investors. However, they are rarely investigated. This study provided quantitative evidence that proves the effectiveness of FWS systems.