R. Wayne Skaggs

MODELING DENITRIFICATION IN TERRESTRIAL AND AQUATIC ECOSYSTEMS AT REGIONAL SCALES

Quantifying where, when, and how much denitrification occurs on the basis of measurements alone remains particularly vexing at virtually all spatial scales. As a result, models have become essential tools for integrating current understanding of the processes that control denitrification with measurements of rate-controlling properties so that the permanent losses of N within landscapes can be quantified at watershed and regional scales. In this paper, we describe commonly used approaches for modeling denitrification and N cycling processes in terrestrial and aquatic ecosystems based on selected examples from the literature. We highlight future needs for developing complementary measurements and models of denitrification. Most of the approaches described here do not explicitly simulate microbial dynamics, but make predictions by representing the environmental conditions where denitrification is expected to occur, based on conceptualizations of the N cycle and empirical data from field and laboratory investigations of the dominant process controls. Models of denitrification in terrestrial ecosystems include generally similar rate-controlling variables, but vary in their complexity of the descriptions of natural and human-related properties of the landscape, reflecting a range of scientific and management perspectives. Models of denitrification in aquatic ecosystems range in complexity from highly detailed mechanistic simulations of the N cycle to simpler source-transport models of aggregate N removal processes estimated with empirical functions, though all estimate aquatic N removal using first-order reaction rate or mass-transfer rate expressions. Both the terrestrial and aquatic modeling approaches considered here generally indicate that denitrification is an important and highly substantial component of the N cycle over large spatial scales. However, the uncertainties of model predictions are large. Future progress will be linked to advances in field measurements, spatial databases, and model structures.

Drainage water management

This article introduces a series of papers that report results of field studies to determine the effectiveness of drainage water management (DWM) on conserving drainage water and reducing losses of nitrogen (N) to surface waters. The series is focused on the performance of the DWM (also called controlled drainage [CD]) practice in the US Midwest, where N leached from millions of acres of cropland contributes to surface water quality problems on both local and national scales. Results of these new studies are consistent with those from previous research reported in the literature that DWM can be used to reduce N losses (primarily in the nitrate nitrogen [NO3-N] form) from subsurface drained fields. The measured impact varied over a wide range (18% to more than 75% reduction in N loss to surface waters), depending on drainage system design, location, soil, and site conditions. Crop yields were increased by DWM on some sites and not on others, with the year-to-year impacts of DWM on yields dependent on weather conditions, as well as the above factors. Papers reporting advances in the development of datasets and models to predict the impact of drainage intensity and DWM on hydrology and water quality at watershed and…

DRAINMOD-FOREST: Integrated modeling of hydrology, soil carbon and nitrogen dynamics, and plant growth for drained forests

We present a hybrid and stand-level forest ecosystem model, DRAINMOD-FOREST, for simulating the hydrology, carbon (C) and nitrogen (N) dynamics, and tree growth for drained forest lands under common silvicultural practices. The model was developed by linking DRAINMOD, the hydrological model, and DRAINMOD-N II, the soil C and N dynamics model, to a forest growth model, which was adapted mainly from the 3-PG model. The forest growth model estimates net primary production, C allocation, and litterfall using physiology-based methods regulated by air temperature, water deficit, stand age, and soil N conditions. The performance of the newly developed DRAINMOD-FOREST model was evaluated using a long-term (21-yr) data set collected from an artificially drained loblolly pine ( L.) plantation in eastern North Carolina, USA. Results indicated that the DRAINMOD-FOREST accurately predicted annual, monthly, and daily drainage, as indicated by Nash-Sutcliffe coefficients of 0.93, 0.87, and 0.75, respectively. The model also predicted annual net primary productivity and dynamics of leaf area index reasonably well. Predicted temporal changes in the organic matter pool on the forest floor and in forest soil were reasonable compared to published literature. Both predicted annual and monthly nitrate export were in good agreement with field measurements, as indicated by Nash-Sutcliffe coefficients above 0.89 and 0.79 for annual and monthly predictions, respectively. This application of DRAINMOD-FOREST demonstrated its capability for predicting hydrology and C and N dynamics in drained forests under limited silvicultural practices.

Hydrology of Poorly Drained Coastal Watersheds in Eastern North Carolina

A 10,000 ha lower coastal plain land near Plymouth in eastern North Carolina has been intensively monitored since 1996 to measure hydro-meteorological parameters including outflows and quality of water drained from fields and subwatersheds with varying land management practices. This study summarized the data for a six-year period (1996-2001) for a 2950 ha forested, a 710 ha agricultural subwatershed and a 8140 ha watershed comprised of agricultural, forested, and riparian lands. The period covered a wide range of weather conditions from a dry year with annual rainfall of 775 mm in 2001 to a wet year with 1512 mm of rain in 1996 with two hurricanes. While 1998 with 1242 mm of annual rain experienced a wet winter and a prolonged dry summer-fall, the conditions were opposite in 1999 (1302 mm of rain) with a dry winter-spring and three hurricanes in the summer and fall. A near normal rainfall (1219 mm of rain) was observed in year 2000. The average annual PET for the site was estimated to be 1000 mm. Variability in annual rainfall was found to have greater effect than the land use type on annual outflows drained from these three watersheds. The average annual runoff/rainfall ratio for the managed pine forest watershed was the lowest compared to two other watersheds. Both the magnitude and frequency of peak flow rates were highest for the agricultural watershed, as expected. Average annual ET, calculated as difference of rainfall and outflow, was 922mm, 714 mm, and 727 mm for forested, agricultural and mixed land use watersheds, respectively. Annual ET estimated by the method suggested by Zhang et al. (2001) were in close agreement with the water balance for all six years when a plant-available water coefficient value of 3.0 was used for the managed pine forest. However, further tests of this ET model are suggested in other watersheds. These results will be valuable for estimating nutrient exports from the watershed as well as verifying watershed scale hydrologic and water quality models.

Development and testing of watershed-scale models for poorly drained soils

Two watershed-scale hydrology and water quality models were used to evaluate the cumulative impacts of land use and management practices on downstream hydrology and nitrogen loading of poorly drained watersheds. Field-scale hydrology and nutrient dynamics are predicted by DRAINMOD in both models. In the first model (DRAINMOD-DUFLOW), field-scale predictions are coupled to the canal/stream routing and in-stream water quality model DUFLOW, which handles flow routing and nutrient transport and transformation in the drainage canal/stream network. In the second model (DRAINMOD-W), DRAINMOD was integrated with a new one-dimensional canal and water quality model. The hydrology and hydraulic routing components of the models were tested using data from a 2950 ha drained managed forest watershed in the coastal plain of eastern North Carolina. Both models simulated the hydrology and nitrate-nitrogen (NO3-N) loading of the watershed acceptably. Simulated outflows and NO3-N loads at the outlet of the watershed were in good agreement with the temporal trend for five years of observed data. Over a five-year period, total outflow was within 1% of the measured value. Similarly, NO3-N load predictions were within 1% of the measured load. Predictions of the two models were not statistically different at the 5% level of significance.

DRAINWAT--BASED METHODS FOR ESTIMATING NITROGEN TRANSPORT IN POORLY DRAINED WATERSHEDS

Methods are needed to quantify effects of land use and management practices on nutrient and sediment loads at the watershed scale. Two methods were used to apply a DRAINMOD-based watershed-scale model (DRAINWAT) to estimate total nitrogen (N) transport from a poorly drained, forested watershed. In both methods, in-stream retention or losses of N were calculated with a lumped-parameter model, which assumes that N concentration decreases exponentially with residence (or travel) time in the canals. In the first method, daily field outflows predicted by DRAINWAT were multiplied by average N concentrations to calculate daily loads at the field edge. Travel time from the field edge to the watershed outlet was computed for each field for each day based on daily velocities predicted by DRAINWAT for each section of the canal-stream network. The second lumped-parameter method was similar but used predicted annual outflow to obtain annual load at the field edge. The load was transported to the watershed outlet, and the in-stream N loss was determined by using a constant average velocity (obtained by long-term DRAINWAT simulations), independent of season, for the entire canal-stream network. The methods were applied on a 2,950 ha coastal forested watershed near Plymouth, North Carolina, to evaluate daily, monthly, and annual export of nitrogen for a five-year (1996-2000) period. Except for some late spring and hurricane events, predicted daily flows were in good agreement with measured results for all five years (Nash-Sutcliffe coefficient, E = 0.71 to 0.85). Estimates of monthly total N load were in much better agreement (E = 0.76) with measured data than were the daily estimates (E = 0.19). Annual nitrogen load was predicted within 17% of the measured value, on average, and there was no difference (. = 0.05) between measured and estimated monthly and annual loads. The estimates of annual N loads using travel time with a daily velocity yielded better results than with the constant average velocity. The estimated delivery ratio (load at the outlet/load at the field edge) for total N was shown to vary widely among individual fields depending on their location in the watershed and distance from the outlet. Both of the methods investigated can potentially be used with GIS in predicting impacts of land management practices on total N loads from poorly drained watersheds.

Temporal variations and controlling factors of nitrogen export from an artificially drained coastal forest.

Nitrogen losses in drainage water from coastal forest plantations can constrain the long term sustainability of the system and could negatively affect adjacent nutrient sensitive coastal waters. Based on long-term (21 years) field measurements of hydrology and water quality, we investigated the temporal variations and controlling factors of nitrate and dissolved organic nitrogen (DON) export from an artificially drained coastal forest over various time scales (interannual, seasonal, and storm events). According to results of stepwise multiple linear regression analyses, the observed large interannual variations of nitrate flux and concentration from the drained forest were significantly (p < 0.004) controlled by annual mean water table depth, and annual drainage or precipitation. Annual precipitation and drainage were found to be dominant factors controlling variations of annual DON fluxes. Temporal trends of annual mean DON concentration could not be explained explicitly by climate or hydrologic factors. No significant difference was observed between nitrogen (both nitrate and DON) export during growing and nongrowing seasons. Nitrate exhibited distinguished export patterns during six selected storm events. Peak nitrate concentrations during storm events were significantly (p < 0.003) related to 30-day antecedent precipitation index and the minimum water table depth during individual events. The temporal variations of DON export within storm events did not follow a clear trend and its peak concentration during the storm events was found to be significantly (p < 0.006) controlled by the short-term drying and rewetting cycles.

Subirrigation System Control for Water use Efficiency

ABSTRACT Afield experiment and a computer simulation analysis were conducted to evaluate the water requirements of subirrigation under three methods of system control. First year results from the field experiment indicate that irrigation water requirements can be reduced by controlling the system such that the midpoint water table depth is allowed to fluctuate within certain limits. A simulation model based on numerical solutions to the Boussinesq equation was developed to allow comparison between the control methods using historical weather records. Several simulations were conducted for each control method to optimize the set points for starting and stopping subirrigation for the given method. Using the optimum set points, the simulations predicted a decreased irrigation requirement when the midpoint water table depth was allowed to fluctuate. The irrigation requirement was decreased by an average of 6.7% over constant water level control for five years of simulations. Much larger differences could have occurred had the set points not been optimized. That is, there is potentially more difference in irrigation water requirements for different set points within a given control method than between two different control methods. The results indicate that there is a good potential for reducing irrigation requirements of subirrigation. A slight modification in the way in which subirrigation systems are currently controlled can result in decreased subirrigation water usage.

Soil Property Changes During Loblolly Pine Production

Three watersheds, each approximately 25 ha, were instrumented to measure and record drainage rate, water table depth, rainfall and meteorological data. Data continuously collected on the site since 1988 include response of hydrologic and water quality variables for nearly all growth stages of a Loblolly pine plantation. Data for drainage outflow rates and water table elevations were used to determine field effective hydraulic conductivity, K, of the profile at various stages of the production cycle. K values of the top 90 cm of the profile for mature plantation forest were 60 to 95 m/day, which are 20 to 30 times the values given in the soil survey for the Deloss series. Harvest did not appear to affect those values, but site preparation for regeneration, including bedding, reduced the effective K to values typically assumed for this series, 3.6 m/d for the top 45 cm and 1.6 m/d for deeper layers.

Incorporating Crop Needs into Drainage System Design

ABSTRACT AN approach is proposed for incorporating crop drainage requirements into drainage design pro-cedures. The overall methodology links crop drainage re-quirements, climatological data, and drainage theory in-to a workable design method through incorporation of the stress-day index concept into the NCSU water management model.