Water quality and hydrology in farm-scale coastal plain watersheds: Effects of agriculture, impoundments, and riparian zones

ABSTRACT:

Dense dendritic stream networks in the Tifton Upland (southeastern U.S. coastal plain) provide an opportunity to determine the effects of land management practices on individual farms on downstream hydrology and water quality. A typical farm will be drained by two or three small streams. The streams may be bordered by riparian forests or impounded into farm ponds. Two adjacent farm-scale basins, both of which were 50 to 60 ha (124 to 148 ac) in size were compared for seven years to determine the effects of upstream agricultural land uses, downstream riparian zones, and small impoundments on stream water quality and hydrology. Stream water quality was sampled at four points on the two basins and at a downstream outlet where the two streams came together in a farm pond. The north basin had more cropland than the south basin and had much more area in plastic-covered beds for vegetable production. The south basin had less land in crop production and much more of the total basin area in farm ponds used to supply irrigation water. The north basin had more surface runoff and higher loads of all nutrients and sediment. The south basin had only 55% of the total runoff of the north basin, probably due to the presence of about 6% of the total watershed area in farm ponds. Up to 26% of the north basin (38% of total cropland) was occupied by wide plastic-covered beds in the last two years of the study. Large quantities of sediment transported in surface runoff from these fields on the north basin led to increases in sediment concentrations and loads of over 100 times compared to both the south basin and to earlier years of the study. Based on reductions in differences in loads and concentrations between upstream and downstream sites, there is less potential to reduce nutrients and sediment once these materials are in stream flow than when water is moving to streams through a riparian buffer. A downstream pond that received inputs from both the north and south basins had significantly lower concentrations of most nutrients and sediments than either of the upstream sampling sites.