Patterns of nitrate transport in an agricultural watershed through consecutive dry years

Abstract

Recently, because of regional climate changes and human activities, the risk and intensity of droughts in the upper and middle Huaihe River Basin, China, have increased. These changes in rainfall may have, in turn, had an influence on the pathways of nitrate transport in this predominantly agricultural watershed. In this study, the characteristics of nitrate transport over consecutive dry years in this watershed were examined using records of streamflow and nitrate concentration data that spanned a period of 12 years (2007-2018) that included 3 consecutive dry years (2011)(2012)(2013). The baseflow was separated from the streamflow using a digital filter method and the nitrate loads were estimated using a regression method. The annual discharge and nitrate load in streamflow and baseflow averaged 23.5 billion m 3 and 41.9 kiloton (kt), and 7.4 billion m 3 and 14.7 kt, respectively. Baseflow contributed more to the total discharge and total nitrate load in the consecutive dry years (41.0% of the total discharge and 56.2% of the total nitrate load) than in wet, normal, and single dry years. Averaged over the whole study period, the monthly baseflow nitrate index (BFNI) was higher than 50% in the dry season and lower than 30% in the flood season. Over the study period, the annual baseflow enrichment ratio (BER) ranged from 0.94 to 1.46 and averaged 1.13, and was highest (1.46) in the consecutive dry years. The results suggest that nitrate was mainly transported to surface water via baseflow during dry conditions and that this process was particularly important during the consecutive dry years. Therefore, to protect surface water, measures should be urgently implemented to control nitrate transport in groundwater during consecutive dry years.