Numerical Simulation And Prediction Of Groundwater Of Large-Scale Irrigation In Northern China

In Northern China, surface water resources cannot meet the growing work; the water needs of the agricultural development and the daily life of the people, so that Groundwater has become the main water resource for industrial, agricultural and domestic usage in this region. It is necessary to evaluate the groundwater resources of the Northern China as an entire aquifer system. Therefore, this research focuses on numerical simulation and prediction of Groundwater, by combining both hydro-climate and groundwater data. The main goals of the study are to simulate and predict groundwater using the Visual MODFLOW-2000(i.e. MODFLOW4.3), as well as reference evapotranspiration software calculation (ETo software) and Excel program. Furthermore, to check the water quality for irrigation purpose in the region.This study presents the results of a mathematical groundwater model developed for the Large-scale irrigation in northern part of China; Baojixia and Yangmaowan irrigation districts (Baoyang irrigation district) in the west of Guanzhong basin; a part of loess plateau, one of the biggest irrigation districts in Shaanxi, and an important agricultural production area in China. It is located in the semi-arid and sub-humid climate.Basic equations of groundwater flow (Darcy’s Law), three-dimensional groundwater flow equation; finite difference method, water balance equations, The FAO Penman-Monteith approach for estimating the reference evapotranspiration (ETo), and the different index methods such as the world health organization standard methods for assessing water quality are developed in this study.The model was calibrated and validated from1981to2009against the historical and observed groundwater data that collected during the study period (the transient simulation with MODFLOW was performed). To achieve the purpose, the data are divided into two periods; the data of first period from1981to2000were used for the calibration purpose, and the available field data during the year2001to2009were used for validation.The water balance was checked for differences between the volume of water that is leaving and entering into the system. The optimization results of parameters were calibrated automatically during the model processing using the Parameter Estimation Package "PEST’(Software).The model was also run to generate groundwater considering three scenarios for a40-year period from2009to2048, taking into account the changing in rainfall due to climate change, urbanization, pumping rates for the different operating well groups in the basin, recharge, evapotranspiration, discharge of irrigation channels, and other parameters such as hydraulic conductivity and specific yield.The following results were drawn after the work was performed:The groundwater of Baoyang Irrigation Aquifer System (BIAS) generally occurred under unconfined conditions. Therefore, the water tables are not stable, fluctuate according to wet and dry seasons, i.e. due to discharge and recharge. The distributed of the water depth is uneven within the region, but as general, the western sides seem to have shallow water depth and the deep water was occurring in the northern parts. In addition, the water level in the north is higher than the south, groundwater flows from north to south, east, and west sides due to the topography of the area (Aquifer profile page44).The results also show that the groundwater level was decreasing in recent years, and will be rose in the autumn season when precipitation and stream stage were high and decreasing during the winter; spring and summer when precipitation and stream stage were low and water demand for agriculture were increasing. The statistical summaries of model calibration, validation and prediction, which performed automatically using MODFLOW such as the correlation coefficient (R)(coefficient of determination R2), Root Mean Square Error (RMSE), Normalized Root Mean Square Error (N.RMSE) and Standard Error of Estimation (SEE) indicated that there is a good fit between estimated and observed values of the observation head. The optimization results of parameters asserted that the horizontal hydraulic conductivity (Kx) and specific yield (Sy) have high sensitivity as well as the recharge, so, they have high influence in the model.According to water balance, there are three major sources of inflow to the system; recharge from precipitation, stream leakage (Irrigation channel) and groundwater storage, while the water outflow consists; total evapotranspiration (ET), stream leakage outflow (irrigation channel), withdrawal from wells, and groundwater outflow (storage outflow). Rainfall is the major source of recharge into the aquifer system during the wet season, and seepage from the irrigation channel (streams) also provides a significant contribution recharge into the system. Major sources of groundwater loss are through direct stream leakage out, the groundwater exploitation, and evapotranspiration.By comparison, the results of three different scenarios that were performed according to climate changes and urbanization, the findings show that the dynamic change in water table depends to the recharge and discharge. For instance, the maximum average drawdown in observation in western part of the area is35.26,44.34, and21.06m in the first, second and third scenario, respectively. Another example in the north part of the area, which has large average value of drawdown in scenario2, is about43.2m, but it was decreased to15.99m in the scenario3.The head elevation in2009of three scenarios is about379.63-615.69m. By the end of the prediction period (i.e.2048), the expected results found that the head elevation were decreased in the range between343.22-604.11m during scenario1,354.76-590.12m, and351.73-604.16m during scenario2and scenario3, respectively. The expected groundwater level, which represented by contour lines are ranging between382.26-595.09m in scenariol,389.05-588.16m, and404.62to589.71m in scenario2and3in the southern and northern parts of the area. The groundwater level may decrease and increase due to different conditions.Forecast results also show that over the prediction period, the highest accuracy is in scenario three. For example, the maximum N.RMSE is4.513%, whereas in scenariol and scenario2is7.001%and8.477%, respectively, these refer to high recharge and storage coefficient during condition three. In spite of, the withdrawn rate and evapotranspiration in scenario3are higher than the other two scenarios; the predicted water head is stable. This stability of water level may due to the large value of the specific yield and hydraulic conductivity; namely the optimization result of Sy and Kx in Scenario3, which provided high recharge and storage.As general, after comparison the results of three different scenarios, one can assert that keeping and sustaining the groundwater in the Baoyang Aquifer need more recharge from either rainfall or artificial recharge such as small dams, water harvesting, diverting from rivers, etc.Water balance and water requirements of some selected crops are calculated on the area in a separate chapter. In this area, the agriculture sector is a major user of water resources, but shortage of precipitation caused an increasing irrigation water demand from groundwater. Therefore, the groundwater was becoming more important of available water. The reference crop evapotranspiration was estimated from1981to2009using the meteorological climate data. The results show that reference, crop evapotranspiration (ETo and ETc) and irrigation water requirements were higher during the dry and hot season than wet and cold season.The groundwater quality of the Baoyang irrigation has been evaluated for suitability of irrigation. In this situation, eighteen groundwater samples were collected from shallow wells in the unconfined aquifer from2002to2009. The various elements of major cations and anions such as sodium (Na+), calcium (Ca2+), magnesium (Mg2+), potassium (K+), Chloride (Cl-), bicarbonate (HCO3), and sulfate (SO42-) were analyzed. In addition, the important chemical parameters such as Total Dissolved Solid (TDS), Sodium Adsorption Ratio (SAR), Soluble Sodium Percent (SSP), Total Hardness (TH), and Residual Sodium Carbonate (RSC) were applied to carry out the assessment of the suitability of water for irrigation purposes. According to the finding of this study, the assessment for irrigation use revealed that the groundwater in the area is poor in some parts and suitable in others. For instance, with respect to SSP, there are two samples have a high sodium percent above60%(62.89%and66.14%) and are not suitable for irrigation. According to the values of TH, the groundwater in the area can be classified as moderately hard and hard water, as well as extremely hard especially in the east part of the region.