Conversion of degraded farmlands to orchards decreases groundwater recharge rates and nitrate gains in the thick loess deposits

Conversion of farmlands to orchards has been reported to greatly influence groundwater recharge and quality; however, it has not been fully discussed in regions with arid climate and thick unsaturated zones. This study attempted to connect land use with groundwater recharge and quality in an arid region covered by loess up to 350 m deep. We collected 1300 soil samples for 26 profiles of 10 m deep, which is deep enough for the water and solutes in soil to be under steady state. We selected sampling sites under farmlands and different deep-rooted fruit trees (i.e., apple and apricot) of varying ages. Based on mass balance methods, we determined the chloride concentrations in soil pore water to estimate the recharge rates, and estimated the groundwater pollutant gains by employing nitrate as pollution index. The mean diffuse recharge was estimated as 3.2 ? 1.5?82.3 ? 44.5 mm yr-1 accounting for 1.7 ? 0.8?13.7 ? 7.4% of mean annual precipitation, and the values under farmlands can be reduced by up to 60% after conversion to deep-rooted fruit trees. The mean groundwater nitrate gains ranged 0.7 ? 0.4?16.5 ? 10.4 kg hm-2 yr-1, and the conversion decreased groundwater nitrate gains by up to 90%. The reduced nitrate pollution to groundwater is different from those regions with shallow vadose zones or humid climate where groundwater pollution is getting more severe with increasing fertilizer application. The reduced recharge was found to be the primary control, but it would extend the lifetime of the risk associated with groundwater pollution from nitrate gains. Our results provide useful information for management of agriculture and groundwater resources in regions with thick vadose zones.