洛惠渠灌区地下水电导率时间稳定性分析

Temporal stability of groundwater electrical conductivity in Luohuiqu irrigation district

土壤盐渍化是当前世界农业发展的重要制约因素,研究地下水电导率的时空变化特征对灌区地下水农业灌溉具有重要指导意义。该研究基于2004-2010年对洛惠渠灌区3个阶地51个观测井地下水电导率的长序列监测结果,利用相对差分法、Spearman秩相关系数法和Morlet小波变换的方法分析了研究区地下水电导率的时间稳定性和周期性特征。结果表明：3个阶地在不同监测时间的电导率平均值均表现为阶地3（海拔〉369-388 m）〉阶地1（海拔〉342-360 m）〉阶地2（海拔〉360-369 m）,且各阶地间地下水电导率的平均值均存在极显著差异（P〈0.01）。3个阶地在不同时间的地下水电导率均属于中等变异,变异系数的范围为44%-75%。井点地下水电导率的时间稳定性强弱与其所在阶地关系不大,不同阶地的地下水电导率均表现出强烈的时间稳定性。阶地1、阶地2和阶地3的地下水电导率代表性位置点分别为40#、38#和45#井点;而整个研究区的地下水电导率代表性位置点为2#井点。基于对36#（高电导率）和43#（低电导率）井点地下水电导率和水位的小波分析表明研究区地下水电导率和水位存在周期性变化。结果表明,洛惠渠灌区地下水电导率具有很强的时间稳定性和周期性,可以利用地下水电导率代表性位置点来监测研究区地下水平均电导率的变化,从而为灌区快速准确地确定灌溉时间以降低土壤盐渍化风险提供一定的参考。

Soil salinization is currently an important limiting factor on agriculture development in irrigation areas all over the world. Electrical conductivity （EC） is one of the main indicators which are used to determine whether the irrigation water will cause soil salinization. It is of important significance to study the temporal and spatial variation of groundwater EC for agricultural irrigation in irrigation districts. This study was conducted to examine the temporal stability and periodicity of groundwater EC in irrigation districts. To accomplish this, 51 observation wells were selected for analysis of groundwater EC in Luohuiqu irrigation district which was an important agricultural production base in Shaanxi Province, China. The 51 observation wells belonged to 3 different terraces which were Terrace 1, 2 and 3. The altitudes of the 3 terraces were 〉342-360, 〉360-369 and 〉370-388 m, respectively. The numbers of observation wells in Terrace 1, 2 and 3 were 26, 13 and 12, respectively. Water samples were collected from 2 m below the well water surface to measure the groundwater EC and temperature at each site. Well 36 （high EC） and 43 （low EC） were selected for long-term observation. A CTD-Diver （USA） groundwater level data logger was adopted for measuring and recording the groundwater EC, temperature and level. According to the long sequence monitoring results of groundwater EC for the 51 observation wells, relative difference analysis and non-parametric Spearman’s rank correlation test were used to check for temporal stability of groundwater EC, while Morlet wavelet analysis was applied to study the periodic variation of groundwater EC and groundwater level of high and low salinity wells. The results indicated that the order of mean groundwater EC values of the 3 terraces at different time instants was Terrace 3 〉 Terrace 1 〉 Terrace 2. Significant difference existed among the mean groundwater EC values of the 3 terraces （P〈0.01）. The groundwater EC values of the 3 terraces at different time instants all indicated moderate spatial variability, with coefficient of variation （CV） values ranging from 44% to 75%. Based on the mean EC value of each observation well, 21 wells were slightly saline and 30 wells were moderately saline. The ranges of minimum and maximum value of groundwater EC were 0.08-0.14 and 0.50-1.00 S/m, respectively. The most of Spearman rank correlation coefficients for groundwater EC were close to 1 （P〈 0.01）, which indicated that the groundwater EC of the 3 terraces exhibited strong temporal stability. But the temporal stability of groundwater EC had little relationship with the terraces which the corresponding well sites belonged to. The representative locations of mean groundwater EC for Terrace 1, 2 and 3 were Well 40、38 and 45, respectively. The representative location of mean groundwater EC for the entire study area was Well 2. Based on the Morlet wavelet analysis of the groundwater EC and water level of Well 36 （high EC） and 43 （low EC）, both of the groundwater EC and water level showed periodic variations. The groundwater EC and water level indicated different vibration periods on different time scales for both Well 36 and 43. The change in groundwater EC on small time scales was mainly affected by lateral recharge sources. The groundwater EC and water level showed an obviously inverse phase on large time scales. In conclusion, the groundwater EC has strong temporal stability and periodicity in Luohuiqu irrigation district. The representative location with mean groundwater EC can be used to monitor the mean groundwater EC of the study area. Irrigation time should be fully considered in irrigation districts to avoid soil salinization. The representative location with mean groundwater EC can provide great guidance to determine the irrigation time quickly and accurately to reduce the risk of soil salinization.