Department of Civil and Environmental Engineering, Texas Tech University, Lubbock, TX, 79409, USA.
Environ Monit Assess. 2018 Feb 21;190(3):157. doi: 10.1007/s10661-018-6476-y.
Seasonal and cyclic trends in nutrient concentrations at four agricultural drainage ditches were assessed using a dataset generated from a multivariate, multiscale, multiyear water quality monitoring effort in the agriculturally dominant Lower Rio Grande Valley (LRGV) River Watershed in South Texas. An innovative bootstrap sampling-based power analysis procedure was developed to evaluate the ability of Mann-Whitney and Noether tests to discern trends and to guide future monitoring efforts. The Mann-Whitney U test was able to detect significant changes between summer and winter nutrient concentrations at sites with lower depths and unimpeded flows. Pollutant dilution, non-agricultural loadings, and in-channel flow structures (weirs) masked the effects of seasonality. The detection of cyclical trends using the Noether test was highest in the presence of vegetation mainly for total phosphorus and oxidized nitrogen (nitrite + nitrate) compared to dissolved phosphorus and reduced nitrogen (total Kjeldahl nitrogen-TKN). Prospective power analysis indicated that while increased monitoring can lead to higher statistical power, the effect size (i.e., the total number of trend sequences within a time-series) had a greater influence on the Noether test. Both Mann-Whitney and Noether tests provide complementary information on seasonal and cyclic behavior of pollutant concentrations and are affected by different processes. The results from these statistical tests when evaluated in the context of flow, vegetation, and in-channel hydraulic alterations can help guide future data collection and monitoring efforts. The study highlights the need for long-term monitoring of agricultural drainage ditches to properly discern seasonal and cyclical trends.
利用在德克萨斯州南部农业主导的下里奥格兰德河谷流域进行的一项多变量、多尺度、多年水质监测工作所产生的数据集，评估了四个农业排水渠中营养物浓度的季节性和周期性趋势。开发了一种创新的基于 bootstrap 抽样的功效分析程序，以评估曼-惠特尼和诺瑟检验辨别趋势的能力，并指导未来的监测工作。曼-惠特尼 U 检验能够检测到在深度较低且不受阻碍的水流处，夏季和冬季营养物浓度之间的显著变化。污染物稀释、非农业负荷和渠道内水流结构（堰）掩盖了季节性的影响。在存在植被的情况下，诺瑟检验检测到周期性趋势的能力最高，主要是针对总磷和氧化氮（亚硝酸盐+硝酸盐），而不是溶解磷和还原氮（总凯氏氮-TKN）。前瞻性功效分析表明，尽管增加监测可以提高统计功效，但效应大小（即在时间序列内的趋势序列总数）对诺瑟检验的影响更大。曼-惠特尼和诺瑟检验都提供了污染物浓度季节性和周期性行为的补充信息，并且受到不同过程的影响。在评估流量、植被和渠道水力变化的背景下，这些统计检验的结果可以帮助指导未来的数据收集和监测工作。该研究强调了对农业排水渠进行长期监测的必要性，以正确辨别季节性和周期性趋势。
