Abstract: Background, aim, and scope Groundwater is an important water resources and a significant factor in maintaining regional environmental health and ecological balance. The combined effect of human activities and climate change has led to changes in the groundwater environment that inevitably have a corresponding impact on groundwater dependent ecosystems .Therefore, an accurate analysis of the dynamic characteristics of groundwater has important practical significance for making rational strategies of groundwater exploitation, the protection of ecological balance and the sustainable development of society and economy. The hypsometric integral is a quantitative index to study the relationship between the horizontal cross-sectional area and its elevation, which is a kind of the quantitative analysis to reflect the status and erosion trend of the geomorphology. It is widely used in the research of geomorphic evolution, evaluation of lithology and tectonics, hydrological characteristics analysis of drainage basin. From the perspective of spatial data model representation of ground objects, both the shallow groundwater surface and the drainage basin topography surface can be described by digital elevation model and have the same mathematical basis for the hypsometric integral analysis. By analyzing the variation characteristics of groundwater surface, the hypsometric integral analysis provides a applicable method for studying groundwater dynamics. Jinghuiqu irrigation district is an important food and vegetable production base in Shaanxi province where residents mainly rely on groundwater. In recent years, the groundwater level and storage in the region has been impacted by industrial and agricultural activities. The study was carried out to analyze the groundwater dynamics in the region, and discuss the role of hypsometric integral in the groundwater dynamic analysis. Materials and methods Shallow groundwater level data in 4 periods of 1978, 1991, 2001 and 2012 was filtered by interval of about 10 a which derived from groundwater level monitoring data of Shaanxi Province Jinghuiqu Authority. The normal distribution condition was verified and the abnormal data were eliminated using QQ-plot tool in the Geostatistical Analyst module of ArcGIS. Gaussian model was applied to fit the semivariogram and the Ordinal Kriging method was used to interpolate the data to get the groundwater surface raster data, covering the study area with a spatial resolution of 100 m for each grid. The descriptive statistical data of the groundwater level in the period of 1978-2012 was analyzed using ArcGIS and the hypsometric integral of each period is calculated. Using Slice tool in ArcGIS Spatial Analysis Tools, the groundwater level in different periods is reclassified to 11 classes. According to the definition of hypsometric curve, the proportion of the total area and the total height in the study area was calculated for the upper limit of each groundwater level class, hypsometric curves of 1978-2012 were depicted using Excel. Results During the period of 1978-2012, the hypsometric integral of groundwater was 0.46, 0.44, 0.38 and 0.39, indicating that the groundwater level and storage was in a decreasing trend. In the period of 1991-2012, the area ratio in the groundwater level of 410-446.19 m decreased to 0, in comparison of 2.54% in 1978. The area in the groundwater level of 342.51-360 m increased continually in decades. The different trends of area ratio in groundwater level of 360-410 m showed the spatial variability of groundwater extraction in different periods. During 1978 and 1991, the area ratio difference in the groundwater level of 360-430 m was fair less, reflecting that the groundwater extraction was stable in the period. Since 2001, the area ratio within the groundwater level range has changed to different extent, reflecting the spatial evolution of groundwater extraction. In the period of 1978-2012, the groundwater storage decreased about 7.08 × 108 m3. Discussion The precipitation, amount of surface water use and groundwater exploitation was the most important factors impacting the groundwater dynamics. The unbalance of recharges and discharges was the main reason for the decrease of groundwater storage in the study area. In the period of 1978~2012, the precipitation showed a downward trend, the effective precipitation infiltration recharge decreased, and evaporation showed an upward trend, the evaporation discharge increased. With the decrease of surface irrigation water intake in the irrigation district, the irrigation infiltration recharge volume decreased from an annual average of 0.92×108 m3 in the 1980s to the current annual average of 0.28×108 m3, which aggravated the decline of the groundwater level. The ratio of irrigation canal irrigation varied drastically and ranged from 0.25 to 1.31. The average values in the 1990s and the first decade of 21st century were 0.90 and 0.83, respectively, which had exceeded the suitable irrigation ratio of 0.35 to 0.7. While the total amount of surface water irrigation declined, the ratio of well irrigation increased significantly, and the depth of groundwater burial increased year by year, indicated that the unbalance of groundwater recharge was increasingly serious. Conclusions The study shows that the hypsometric curve is useful to depict the distribution characteristics of groundwater level change and storage variation, and the amount of groundwater storage variation can be derived from the hypsometric integral value. It proves that the hypsometric integral is applicable in the groundwater dynamic analysis. Recommendations and perspective While using the hypsometric integral to analyze the groundwater dynamics, the study area could be divided into smaller sub-areas, and the hypsometric integral value of each sub-area would be calculated separately to depict the spatial distribution characteristics of groundwater level and storage.
Keywords: hypsometric integral; groundwater; dynamic; analysis; Jinghuiqu irrigation district