Patterns and associations between dominant crop productions and water quality in an irrigated watershed

Authors: Nouwakpo, Sayjro; Bjorneberg, David - Dave; Rogers, Christopher

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2023
Publication Date: 11/1/2023
Citation: Nouwakpo, S.K., Bjorneberg, D.L., Rogers, C.W. 2023. Patterns and associations between dominant crop productions and water quality in an irrigated watershed. Journal of Soil and Water Conservation. 78(6):466-478. . https://doi.org/10.2489/jswc.2023.00176.
DOI: https://doi.org/10.2489/jswc.2023.00176

Interpretive Summary: Irrigation consumes vast amounts of freshwater but can also pollute remaining water resources with excess agricultural nutrients and sediments. With increasing competition for freshwater resources, improving irrigation efficiency is needed. In this paper, long-term monitoring data collected as part of the Conservation Effect Assessment Project (CEAP) were used to understand how various crops impact water quantity and quality. The monitoring data were collected in the Twin Falls Canal Company irrigation tract, an irrigated watershed of the Northwest United States drawing its water from the Snake River and where a diversity of crops are cultivated. Results of the study show that tight water management under potato cultivation was associated with more water returning to the Snake River while late season water demand from winter-planted small grains led to more water used for irrigation during fall seasons. Crops favoring early or yearlong ground cover such as alfalfa, pasture or sugar beets were associated with benefits in soil loss reduction. The reduction in excess nutrients such as phosphorus and nitrates in water returning to the Snake River was linked to multiple crops and likely a function of factors such as crop nutritional demand, erosion-reducing management actions and the reemergence of groundwater in surface water supplies. Findings of this study are expected to inform more detailed experimental research on the link between management of specific crops and water quantity and quality at the watershed scale.

Technical Abstract: Irrigation consumes the largest share of freshwater resources but is a necessary practice to boost agricultural output to meet increasing global demand for food and fiber. Irrigation not only impacts water quantity but can also degrade water quality. Research efforts have explored various aspects of irrigation efficiency and irrigated crop productivity, but few studies have examined how different crops collectively modulate water utilization and water quality at the watershed scale. In this study long-term water quantity and quality monitoring data collected as part of the Conservation Effect Assessment Project (CEAP) combined with crop and evapotranspiration (ET) modeling products were used to elucidate relationships between crop and water processes in an irrigated watershed. We use a correlational approach to build relationships between water quantity and quality metrics and the fractional volumes of ET associated with major crops in the Twin Falls Canal Company irrigation tract. Results suggest that sub-watershed size and subsurface flow contribution in drainage tunnels influenced hydrologic patterns observed and led to 2 distinct groups. Group 1 sub-watersheds were large, typically included subsurface drain tunnels and had high return flow volumes and low sediment concentration while group 2 sub-watersheds were smaller in size, had low return flow volumes and high sediment concentration. Irrigation return flow volume normalized by sub-watershed area was positively associated with ET fractions of potato (Solanum tuberosum) in group 1 during the spring and summer months. Spring sediment loss per return flow volume showed a negative association with ET fractions of sugar beet and combined alfalfa (Medicago sativa) and pasture crops in group 2. A negative association was found between phosphorus (P) load per return flow volume and ET fractions of alfalfa / pasture, corn (Zea mays), dry beans (Phaseolus vulgaris), and sugar beet (Beta vulgaris) across sub-watershed groups. Nitrate (NO3-N) load per return flow volume was negatively associated with potato and corn ET fractions in group 1 especially during the spring and fall month but positively associated with dry beans over the irrigation season. While direct cause and effect were not established between crops and water quantity and quality, results from this study provide valuable information on management factors associated with various crop production systems that may control observed hydrologic response. Example of factors considered in explaining some of the observed patterns include early germination and ground coverage, tight control on soil water content, and the erosion attenuation effect of sedimentation ponds.