Metamodels and Nonpoint Pollution Policy in Agriculture

Informed debate on agricultural nonpoint pollution requires evaluation of regional water quality in relation to management practices. It is prohibitive, in terms of cost and time, to run the site-specific process models for regional policy analysis. Therefore, a simplified and robust technique--metamodeling--is suggested to evaluate regional water quality. Data from an experimentally designed simulation of complex surface water and groundwater process models, PRZM and STREAM, are used to develop statistically validated metamodels. The estimated metamodels were integrated with a regional agricultural economic decision making model to evaluate the surface water and groundwater loadings of 16 major corn and sorghum herbicides. Spatial probability distributions are derived for herbicide concentrations exceeding the toxicity-weighted benchmark from the EPA. We estimate that 1.2 percent of the regional soils will lead to groundwater detection of atrazine exceeding 0.12 ?/L, which compares well with the findings of the EPA\u27s groundwater monitoring survey. We find no-till practices to significantly reduce the surface water concentration of atrazine and other herbicides with less impact on groundwater contamination suggesting indirect gains to soil conservation policies. But we also note that an atrazine ban could lead to increased soil erosion, even with the conservation compliance provisions fully incorporated

Assessment of three long-term gridded climate products for hydro-climatic simulations in tropical river basins

10.3390/w9030229Water (Switzerland)9322

Interplay between spatially explicit sediment sourcing, hierarchical river-network structure, and in-channel bed material sediment transport and storage dynamics

Understanding how sediment moves along source to sink pathways through watersheds„from hillslopes to channels and in and out of floodplains„is a fundamental problem in geomorphology. We contribute to advancing this understanding by modeling the transport and in-channel storage dynamics of bed material sediment on a river network over a 600æyear time period. Specifically, we present spatiotemporal changes in bed sediment thickness along an entire river network to elucidate how river networks organize and process sediment supply. We apply our model to sand transport in the agricultural Greater Blue Earth River Basin in Minnesota. By casting the arrival of sediment to links of the network as a Poisson process, we derive analytically (under supply-limited conditions) the time-averaged probability distribution function of bed sediment thickness for each link of the river network for any spatial distribution of inputs. Under transport-limited conditions, the analytical assumptions of the Poisson arrival process are violated (due to in-channel storage dynamics) where we find large fluctuations and periodicity in the time series of bed sediment thickness. The time series of bed sediment thickness is the result of dynamics on a network in propagating, altering, and amalgamating sediment inputs in sometimes unexpected ways. One key insight gleaned from the model is that there can be a small fraction of reaches with relatively low-transport capacity within a nonequilibrium river network acting as ñbottlenecksî that control sediment to downstream reaches, whereby fluctuations in bed elevation can dissociate from signals in sediment supply. ©2017. American Geophysical Union. All Rights Reserved