R. W. Skaggs

Hydrologic and water quality impacts of agricultural drainage

Abstract While some of the worlds most productive agriculture is on artificially drained soils, drainage is increasingly perceived as a major contributor to detrimental off‐site environmental impacts. However, the environmental impacts of artificial or improved agricultural drainage cannot be simply and clearly stated. The mechanisms governing the hydrology and loss of pollutants from artificially drained soils are complex and vary with conditions prior to drainage improvements and other factors: land use, management practices, soils, site conditions, and climate. The purpose of this paper is to present a review of research on the hydrologic and water quality effects of agricultural drainage and to discuss design and management strategies that reduce negative environmental impacts. Although research results are not totally consistent, a great majority of studies indicate that, compared to natural conditions, drainage improvements in combination with a change in land use to agriculture increase peak runof...

A comparison of the watershed hydrology of coastal forested wetlands and the mountainous uplands in the Southern US

Abstract Hydrology plays a critical role in wetland development and ecosystem structure and functions. Hydrologic responses to forest management and climate change are diverse in the Southern United States due to topographic and climatic differences. This paper presents a comparison study on long-term hydrologic characteristics (long-term seasonal runoff patterns, water balances, storm flow patterns) of three watersheds in the southern US. These three watersheds represent three types of forest ecosystems commonly found in the lower Atlantic coastal plain and the Appalachian upland mountains. Compared to the warm, flat, and shallow groundwater dominated pine flatwoods on the coast, the inland upland watershed was found to have significantly higher water yield, Precipitation/Hamons potential evapotranspiration ratio (1.9 for upland vs 1.4 and 0.9 for wetlands), and runoff/precipitation ratio (0.53±0.092 for upland vs 0.30±0.079 and 0.13±0.094 for wetlands). Streamflow from flatwoods watersheds generally are discontinuous most of the years while the upland watershed showed continuous flows in most years. Stormflow peaks in a cypress–pine flatwoods system were smaller than that in the upland watershed for most cases, but exceptions occurred under extreme wet conditions. Our study concludes that climate is the most important factor in determining the watershed water balances in the southern US. Topography effects streamflow patterns and stormflow peaks and volume, and is the key to wetland development in the southern US.

Field Evaluation of a Water Management Simulation Model

ABSTRACT THE water management simulation model, DRAINMOD, was tested using field data for over 5 yr of record from three locations in the NC Coastal Plains. Each site had field scale drainage systems with provisions for subirrigation and controlled drainage. Three soil types and five different drainage system designs were included in the experiment from which 21 site-yr of data were obtained. Rainfall intensity and water table elevations were measured continuously at each site and the observed day end water table elevations were compared to predicted values. Effective lateral hydraulic conductivity values were measured in the field using both auger hole and water table drawdown methods. Numerous other field and laboratory mea-surements were made for each soil to determine input soil property and site parameter data. Comparison of predicted and measured water table elevations were in excellent agreement with the daily water table depths having standard errors of estimate ranging from 7.5 to 19.6 cm. The average absolute devia-tion between predicted and observed water table depths for 21 site-years of data (approximately 7400 pairs of daily predicted and measured values) was only 8.1 cm. Based on the results of the study, DRAINMOD can be reliably used to predict the effect of drainage system design on water table elevations.

DRAINMOD-N, A NITROGEN MODEL FOR ARTIFICIALLY DRAINED SOIL

DRAINMOD-N, a quasi two-dimensional model that simulates the movement and fate of nitrogen in shallow water table soils with artificial drainage, is described. Results of sensitivity analyses are presented and model predictions are compared with results from VS2DNT, a more complex, two-dimensional model. The nitrogen transport component is based on an explicit solution to the advective-dispersive-reactive (ADR) equation. Nitrate-nitrogen is the main N pool considered. Functional relationships are used to quantify rainfall deposition, fertilizer dissolution, net mineralization, denitrification, plant uptake, and surface runoff and subsurface drainage losses. A sensitivity analysis showed DRAINMOD-N predictions are most sensitive to the standard rate coefficients for denitrification and mineralization and nitrogen content in rainfall. Simulated daily water table depths were within 0.121 m, cumulative subsurface drainage rates were within 0.016 m, and cumulative surface runoff rates were within 0.003 m, of those predicted by VS2DNT for a 250-day period. DRAINMOD-N predictions for NO3-N losses in subsurface drainage water only differed from VS2DNT predictions by less than 2.6 kg ha–1. DRAINMOD-N predictions for denitrification were within 8%, for plant uptake were within 15%, and for net mineralization were within 26%, of those simulated by VS2DNT.

EFFECTS OF CONTROLLED DRAINAGE ON THE HYDROLOGY OF DRAINED PINE PLANTATIONS IN THE NORTH CAROLINA COASTAL PLAIN

Abstract This paper presents results of a 5 year study to characterize the hydrology (rainfall, interception, evapotranspiration (ET), soil water storage, drainage rate, lateral seepage, and water table fluctuations) of three identical drained, pine-forested watersheds in Carteret County, North Carolina. During the 2 year calibration period (1988–1989), all three watersheds were operated in conventional drainage mode with the weirs in the outlet ditch approximately 1.0 m below the soil surface. About 17% of the total rainfall was intercepted and subsequently evaporated and 53% was removed by transpiration and evaporation from the soil during this period. Drainage removed about 28% and the remaining 3% was lost by lateral seepage. During the 3 year controlled drainage treatment period (1990–1993), drainage in Watershed 2, managed for tree growth, was reduced to 21% of gross rainfall as compared with 30.5% for Watershed 1 under free drainage. Watershed 3, managed to minimize offsite impacts, yielded 26% of gross rainfall as drainage. Interception loss accounted for about 14.5% of the gross rainfall. ET amounts computed as the residual in a water balance, were 50%, 60%, and 55% of total rainfall for Watersheds 1, 2, and 3, respectively. The effects of controlled drainage on water table depths, drainage and ET were demonstrated for seasonal and year-to-year variation in rainfall. The controlled drainage treatments affected both drainage volumes and daily peak outflow rates. The treatment in Watershed 3 was more effective in reducing peak outflow rates.

THE NITROGEN SIMULATION MODEL, DRAINMOD-N II

DRAINMOD-N II is a field-scale, process-based model that was developed to simulate nitrogen dynamics and turnover in the soil-water-plant system under different management practices and soil and environmental conditions. It is an enhanced version of the nitrogen (N) simulation model, DRAINMOD-N, that simulates a more complete N cycle, adds a carbon (C) cycle, and operates at different levels of complexity. Processes considered in the model include atmospheric deposition, application of mineral N fertilizers including urea and anhydrous ammonia (NH3), soil amendment with organic N (ON) sources including plant residues and animal waste, plant uptake, organic C (OC) decomposition and associated N mineralization/immobilization, nitrification, denitrification, NH3 volatilization, and N losses via subsurface drainage and surface runoff. Nitrogen pools considered in the model are nitrate-nitrogen (NO3-N), ammoniacal nitrogen (NHx-N) and ON. DRAINMOD-N II includes a submodel that simulates C dynamics in the soil-plant system using a C cycle similar to that of the CENTURY model. A simplified approach is used to simulate temporal changes in soil pH; consequently, the model determines the composition of the NHx-N pool and, if necessary, changes its operation mode. DRAINMOD-N II simulates N reactive transport using a finite difference solution to a multiphase form of the one-dimensional advection- dispersion-reaction equation. Model output includes daily concentrations of NO3-N and NHx-N in soil solution and drain flow, daily OC content of the top 20 cm soil layer, and cumulative rates of simulated N processes.

DRAINMOD: Model Use, Calibration, and Validation

DRAINMOD is a process-based, distributed, field-scale model developed to describe the hydrology of poorly drained and artificially drained soils. The model is based on water balances in the soil profile, on the field surface, and, in some cases, in the drainage system. This article briefly describes the model and the algorithms that are used to quantify the various hydrologic components. Inputs for soil properties, site parameters, weather data, and crop characteristics required in the application of the model are presented and discussed with respect to their role in calibration. Methods for determining field effective values of key inputs to the model, either independently or as a part of the calibration process, are demonstrated in a case study. The case study involved calibrating DRAINMOD with two years of field data for a subsurface drained agricultural field in eastern North Carolina, followed by testing or validation of the model with two additional years of data. Performance statistics indicated that the model with calibrated input data accurately predicted daily water table depths with Nash-Sutcliffe modeling efficiency (EF) values of 0.68 and 0.72, daily drainage rates (EF = 0.73 and 0.49), and monthly drainage volumes (EF = 0.87 and 0.77) for the two-year validation period.

Effect of Drainage System Design and Operation on Nitrate Transport

ABSTRACT THE computer simulation model, DRAINMOD, was modified to predict nitrate movement from artificial-ly drained soils with high water tables. Nitrate concentra-tions in surface runoff, subsurface drainage and seepage waters were assumed to be constant and independent of the drainage system design for a corn-soybean rotation on these high water table soils. Total annual nitrate outflow was determined for alternative drainage system designs and operational procedures. The results showed that trafficability and crop protection requirements can be satisfied by several different drainage system designs. For the poorly drained soil considered in this study, there was a three-fold difference in NO3-N outflow among systems that satisfied drainage objectives. The amount of nitrate that leaves the field through drainage waters can be reduced by using controlled drainage during the winter months and during the growing season. However if the controlled drainage systems are not used properly an overall increase in nitrate outflow may result.

DRAINMOD MODIFICATIONS FOR COLD CONDITIONS

The field hydrology model DRAINMOD was modified to include freezing and thawing, and snowmelt components. Based on daily hydrologic predictions of the original model, the modified DRAINMOD numerically solves the heat flow equation to predict soil temperature. When freezing conditions are indicated by below zero temperatures, the model calculates ice content in the soil profile and modifies soil hydraulic conductivity and infiltration rate accordingly. Recorded precipitation is separated as rain and snow when daily average air temperature is above or below a rain/snow dividing base temperature. Snow is predicted to accumulate on the ground until air temperature rises above a snowmelt base temperature. Soil surface temperature is recalculated when snow cover exists. Daily snowmelt water is added to rainfall, which may infiltrate or run off depending on soil freezing condition. The modified DRAINMOD predictions of soil temperature agreed well with field observations at Plymouth, North Carolina, Truro, Nova Scotia, and Lamberton, Minnesota. Assuming air temperature as the soil surface boundary condition increased the variability of soil temperature predictions at shallow depths, agreement with field measurements was still good. The method of using average air temperature as an indicator to separate snow and rain worked very well for Carsamba, Turkey. At Truro, Nova Scotia, however, the method was not as successful, and several snow events were predicted as rainfall and vice versa. Compared with the original version of DRAINMOD, the modified version predicts fewer drainage flow events in winter months because of snow accumulation on the surface. Subsurface drainage and/or surface runoff resulting from snowmelt are predicted when air temperature rises, the snow melts, and the soil begins to thaw.

Effects of Subsurface Drain Depth on Nitrogen Losses from Drained Lands

A simulation study was conducted to determine effects of drain depth on nitrogen (N) loss in drainage water. Simulations were conducted for drain depths of 0.75, 1.0, 1.25, and 1.5 m for a Portsmouth sandy loam at Plymouth, North Carolina. A wide range of drain spacing was considered for each depth. Corn yields were predicted and an economic analysis was conducted to determine the drain spacing giving maximum predicted profit for each depth. Results showed that nitrogen losses from subsurface drains can be reduced by placing the drains at shallow depths. In order to satisfy agricultural production requirements, shallow drains must be placed closer together than deeper drains. While predicted agricultural profits for the shallow drains are reduced somewhat compared to the deeper drains, overall profits are substantially increased when the cost of removing N from drainage water is considered.