Transport Patterns of Soil Runoff Phosphorus in a Watershed Under Variable Stream Discharge

Abstract:

Temporal and spatial distribution of sediments and agricultural nutrients were studied in Cedar River watershed of northeast Iowa. The watershed extends its headwater from southern Minnesota and flows together with Iowa River in Conesville, Iowa where it joins the Mississippi River. The watershed, which is predominantly agricultural, covers a drainage area of 7,815 mi². Phosphorus is one of the nutrients that cause eutrophication in Iowa’s surface water bodies. The significant build-up of phosphorus in Iowa’s stream sediments is linked to developing a zone of hypoxia as far away as the Gulf of Mexico. A comprehensive study of nutrient distribution, transport, and its dynamic relationships with the watershed and the surrounding environment is an urgent need at the moment. The study shows a considerably higher loads of total suspended sediments (TSS) during the growing season as compared to the baseline. Water and sediment samples were collected from 18 sites. Most sites show initial high loads of TSS in April and early May and then it appears to peak again in late June and July. The first peaks of TSS can be attributed to the snow melt episodes causing soil loss from the agricultural fields that are not adequately covered by crops. TSS peaks again during mid-summer, which is attributed to rain events, causing high loads of soil runoff phosphorus in the stream water. As an example, the study area had a 5.02 inches of rainfall from June 16^th through the 19^th. The data collected on June 21^st showed a jump in TSS concentration from 30 mg/L to 71 mg/L in the 9 sites toward the highly farmed northern part of the study area. The average increase toward the southern part of the area was from 23 mg/L to 122 mg/L. The total load of phosphorus also increased with the high loads of suspended solids in the stream. The direct correlation between total phosphorus and TSS in the watershed is observed until the end of July. This reiterates the importance of soil conservation practices to achieve the ultimate goals of nutrient reduction, especially phosphorus.