Integrated assessment modeling reveals near-channel management as cost-effective to improve water quality in agricultural watersheds

Abstract

Despite decades of policy that strives to reduce nutrient and sediment export from agricultural fields, surface water quality in intensively managed agricultural landscapes remains highly degraded. Recent analyses show that current conservation efforts are not sufficient to reverse widespread water degradation in Midwestern agricultural systems. Intensifying row crop agriculture and increasing climate pressure require a more integrated approach to water quality management that addresses diverse sources of nutrients and sediment and off-field mitigation actions. We used multiobjective optimization analysis and integrated three biophysical models to evaluate the cost-effectiveness of alternative portfolios of watershed management practices at achieving nitrate and suspended sediment reduction goals in an agricultural basin of the Upper Midwestern United States. Integrating watershed-scale models enabled the inclusion of near-channel management alongside more typical field management and thus directly the comparison of cost-effectiveness across portfolios. The optimization analysis revealed that fluvial wetlands (i.e., wide, slow-flowing, vegetated water bodies within the riverine corridor) are the single-most cost-effective management action to reduce both nitrate and sediment loads and will be essential for meeting moderate to aggressive water quality targets. Although highly cost-effective, wetland construction was costly compared to other practices, and it was not selected in portfolios at low investment levels. Wetland performance was sensitive to placement, emphasizing the importance of watershed scale planning to realize potential benefits of wetland restorations. We conclude that extensive interagency cooperation and coordination at a watershed scale is required to achieve substantial, economically viable improvements in water quality under intensive row crop agricultural production.

Existing wetland conservation programs miss nutrient reduction targets

“…Of all nitrate exported from soil in our study, the highest fraction intercepted by restored wetlands was 16% for full adoption of the WRP (Figure 3). In contrast, our analysis of FWP and WRP restoration predicts greater nitrate removal from wetland restoration than integrated assessment modeling (<2%) 8,11 , or biophysical modeling (12%) 19 when wetland restoration occurs only on non-tile-drained land. The wide range in previous estimates for the efficacy of interventions reflect the importance of comprehensively accounting for the entire watershed system in estimating restoration outcomes, not just isolated components.…”

“…Restored fluvial wetlands that exchange with river flows during downstream transport, and potentially intercept groundwater and river flux from all upstream croplands may be more effective at removing nitrogen than cropmargin wetlands at watershed scales 11…”

“…. Policies should therefore include interventions that focus on fluvial wetlands11 or floodplain restoration 52 , while maintaining dialogue with stakeholders to address how conservation measures benefit and impact local communities and other ecosystem services provided by the watershed 53 . As we consider of the breadth of nature-based nutrient reduction strategies, we will need mechanistic evaluations of theZuidema et al (In Prep) role of temporal dynamics, watershed position, and future changes in both climate and cropping practices to ensure feasibility of these approaches.…”

Disconnectivity matters: the outsized role of small ephemeral wetlands in landscape-scale nutrient retention

“…At the landscapescale, N retention potential of wetlandscapes can be quantified as a function of N retention in individual wetlands, as well as the distribution and connectivity of wetlands across the landscape. Hansen et al used a coupled model to explore N retention dynamics in the Le Sueur River Basin in the Minnesota River basin, and found restoration of floodplain wetlands to be the most cost-effective strategy for N retention [17]. Evenson et al found that restoring 2% of the area of the Upper Mississippi River Basin to wetlands can reduce the outlet N loads by 12% [18].…”

Nutrient Pollution and U.S. Agriculture: Causal Effects, Integrated Assessment, and Implications of Climate Change

“…The case of wetlands deserves some additional attention as recent work has shown that fluvial wetlands, which are connected to rivers and streams, are particularly effective in removing legacy nitrogen too. Hansen et al (2021) apply an integrated assessment model to optimize nutrient policy in the Le Sueur River Basin (LSRB) of the Upper Mississippi and conclude that shallow fluvial wetlands are the most cost effective approach for reducing nitrate and sediment loads. The annualized installation and management costs are about $600 per acre per year (/a/y).…”