Transport and transformation of nutrients and sediment in two agricultural watersheds in northeast Arkansas

Authors: ARYAL, NIROJ - Orise Fellow; Reba, Michele

Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2016
Publication Date: 1/2/2017
Publication URL: https://handle.nal.usda.gov/10113/5661758
Citation: Aryal, N., Reba, M.L. 2017. Transport and transformation of nutrients and sediment in two agricultural watersheds in northeast Arkansas. Agriculture, Ecosystems and Environment. 236:30-42.

Interpretive Summary: The Mississippi River Valley Alluvial Aquifer (MRVAA) or Delta Region of Arkansas produces major quantity of row crops annually contributing to State Agriculture that ranked in top 25 in production of 23 commodities among states in the USA in 2013. Despite the importance and contribution of the agriculture in the region, nutrients and sediment loss from fields reduce productivity and impact the environment adversely, most well-known of which is hypoxia in the Gulf of Mexico. This paper contributed to increasing understanding transport and transformation of nutrients and sediment at small tributary streams, a major source of pollutant loads to the Mississippi River and the Gulf of Mexico. At two small agricultural watersheds in Northeast Arkansas, nutrients and sediment were measured at five locations per watershed. The results of the research indicated that concentrations of nutrients (except ammonium) and sediment remained constant or increased as the water flowed downstream leading to increased loads. Additionally, pollutants concentration and loads were high in spring (May-June) and late fall (Nov-Dec). The results implied nutrients and sediment reduction strategies need to target control of pollution at the field during spring and late fall.

Technical Abstract: Agriculture is vital to Arkansas economy as it contributes $20 billion annually, double the average national contribution to the state GDP. Arkansas is ranked in the top 5 in rice, cotton seed and sorghum, and top 20 in soybean, corn for grain, and wheat production nationally. Despite the importance of agriculture, nutrients and sediment losses from fields add economic burden to farmers and negative impacts on the environment, most well-known of which is hypoxia in the Gulf of Mexico. Concerted efforts in understanding nutrients and sediment transport at different spatial scales among others is immediately needed to develop a sustainable solution to the water quality problems. We evaluated nutrients and sediment transport and transformation at two small agricultural watersheds, Little River Ditches Basin (LRDB) and Lower St. Francis Basin (LSFB), in Northeast Arkansas to fill the gap between field scale and larger watersheds. Flow, nutrients, and sediment were measured at 3-5 locations per watershed in these two contrasting watersheds. Turbidity increased considerably at LSFB and remained constant at LRDB as the water traveled downstream. The median nitrate-N concentration at LRDB increased from 1.64 to 2.34 mg L-1 as the watershed size increased, in contrast to no increase at LSFB. Nitrate-N were high in spring and late fall at both watersheds. TP and SRP concentrations remained constant, but ammonium-N decreased as the water travelled downstream in both watersheds. The pollutant loads increased as the water flowed downstream. The annual loss of nitrogen was 9.6 and 8.6 kg ha-1, sediment was 1604 and 1958 kg ha-1, and SRP was 0.8 and 0.9 kg ha-1, respectively from LRDB and LSFB. These results are useful to help inform land management in these watersheds.