M. C. Hirschi

Relationship Between Soil Moisture Content and Soil Surface Reflectance

Depending on the topography and soil characteristics of an area, soil moisture, an important factor in crop productivity, can be quite variable over the land surface. Thus, a method for determination of soil moisture without the necessity for exhaustive manual measurements would be beneficial for characterizing soil moisture within a given region or field. In this study, soil surface reflectance data in the visible and near-infrared regions were analyzed in conjunction with surface moisture data in a field environment to determine the nature of the relationship between the two, and to identify potential methods for estimation of soil moisture from remotely sensed data in these wavelengths. Results indicate that it is feasible to estimate surface (0 to 7.6 cm) soil moisture from visible and near-infrared reflectance, although estimating moisture regimes rather than precise water content is perhaps more likely. Furthermore, an exponential model was appropriate to describe soil moisture from spectral reflectance data. In particular, the visible region of the electromagnetic spectrum works well with such a model. A partial least squares analysis with improved R2 values over the single-band models indicated that mulitspectral data may add more useful information about soil moisture as compared to single-band data. The results also suggested that the performance of reflectance models for moisture estimation is a function of soil types; the estimation results were better for the lighter of the two soils in this study.

NITRATE IN RIVER AND SUBSURFACE DRAINAGE FLOWS FROM AN EAST CENTRAL ILLINOIS WATERSHED

The objective of this study was to quantify the effects of crop management systems on the movement of nitrate to a drinking water reservoir in a watershed with intensive row-crop agriculture and sub-surface drainage in eastern Illinois (USA). Nitrate concentrations and sub-surface tile flow were monitored for six years at fields with various tillage and cropping management practices; the eight tile systems drained areas varying from 3 to 21 ha. Nitrate concentrations were also determined at various locations along the main stream of the watershed at locations that defined watershed sizes from 6910 to 48 900 ha. Concentrations of nitrate-N in the river followed a pronounced seasonal cycle, with maximum concentrations between 10 and 15 mg/L occurring in the spring and minimum concentrations of 0 to 5 mg/L occurring in the autumn. There were no significant differences in nitrate concentrations among specific sampling locations along the river, suggesting that the entire watershed is contributing more or less equally to the nitrate load. Nitrate-N concentrations from the tile drains varied from 0 to 39 mg/L and differed among field and cropping management systems used. Seasonal variations of nitrate concentrations in tile drain effluent were not as well defined as observed in the river. Nitrate-N concentrations in tile drain effluent were higher from fields where greater amounts of nitrogen fertilizer were applied, particularly when the fertilizer was applied prior to planting. Pre-plant application systems with average nitrogen application of 107 kg-N/ha/yr yielded a mean concentration of nitrate-N of 16.8 mg/L in the tile drain outflow which was significantly greater than the mean concentration of nitrate-N of 10.2 mg/L from the side dress and manure application systems that had received an average nitrogen application of 93 kg-N/ha/yr. The mean concentration of nitrate-N in tile drain discharge from continuous grass was 1.0 mg/L. Nitrate-N losses from cropped fields ranged from 14 to 35 kg/ha/yr depending upon the management system, which were 14 to 36% of the nitrogen applied. Losses from the grassed system were 3.8 kg/ha/yr and at the most upstream river station were 11 kg/ha/yr of nitrate-N.

Kentucky Rainfall Simulator

ABSTRACT THE construction and operating characteristics of the Kentucky Rainfall Simulator, a new, highly portable rainfall simulator, is described. It is a nozzle type simulator and is suitable for both large- and small-scale studies of the erosion, infiltration, and runoff processes. The simulator closely approximates the kinetic energy of natural rainfall at intensities greater than 25 mm/h. Rainfall intensities ranging from 3.5 to 185 mm/h can be produced, and measured uniformity coefficients range from 80.2 to 83.7. Runoff and sediment sampling devices used with the Kentucky Rainfall Simulator are also described.

SENSITIVITY ANALYSIS OF THE ROOT ZONE WATER QUALITY MODEL

The long-term goal of this research is to improve tools for predicting water quality in agricultural watersheds with significant sub-surface drainage. Specifically, USDA-ARS’ Root Zone Water Quality Model (RZWQM) was investigated as a tool for predicting the effects of agricultural management practices on nitrate loads in tile drainage in east central Illinois. Sensitivity of the model was explored to identify those input parameters with the greatest influence on tile flow, nitrate in tile drainage, and crop yield. Studies were made within the contexts of two hydraulic descriptions of a silty clay loam soil and two crops, corn and soybeans. Simulated tile flow proved to be most sensitive to drain spacing and soil hydraulic properties. In addition to the parameters affecting tile flow, tile nitrate-N was also greatly influenced by soil macroporosity. Biomass required to achieve a leaf area index of 1.0 had the greatest impact on crop yield. More information is needed regarding the statistical distributions of the input parameters. If the model is to be used to predict nitrate loads in tile drainage, particular care should be taken in selecting saturated, lateral hydraulic conductivity and Brooks-Corey soil properties.

Subsurface Drainage and Water Quality: The Illinois Experience

Drainage of excess water from agricultural lands is essential for crop growth. In the Midwestern U.S., Illinois, Indiana, Iowa, Ohio, Minnesota, Michigan, Missouri, and Wisconsin are some of the states that are highly drained with intensive subsurface drainage systems. Illinois alone has a total drained area of approximately 4 million ha. Many watersheds in east-central Illinois have less than 1% surface gradient and poorly drained soils, yet subsurface drains have made these lands some of the most productive farmlands in the world. Subsurface drainage enhances productivity and reduces sediment transport and phosphorous losses from fields; however, it increases NO3-N delivery to the receiving water bodies. Nitrate-N is mobile and can be lost from the soil profile by leaching and through subsurface drains (tile drains). Nitrate-N concentrations in drinking water reservoirs in many Midwestern states frequently exceed the EPAs maximum contaminant level (MCL) of 10 mg L-1. Several studies show NO3-N concentration data of more than 10 mg L-1 in discharges from subsurface drains. This article presents a history of subsurface drainage in Illinois, summarizes the results of decade-long field and watershed-scale research on subsurface drainage, and provides information on some innovative practices that are currently being evaluated to meet water quality challenges. It has been observed that infiltration is the predominant hydrologic process, and surface runoff rarely occurs from these watersheds. It has also been observed that base flow contributes significantly higher nutrient loadings into the streams than the subsurface drains from these watersheds.

KYERMO—A Physically Based Research Erosion Model Part I. Model Development

ABSTRACT AN event-based erosion-model, known as KYERMO, was developed as a research tool to isolate important sub-processes within the overall erosion process. The model was developed using physically-based relation-ships to allow its use for evaluating simpler procedures for erosion estimation. Model development is described in this paper, with the results of sensitivity analyses and testing to be described in a subsequent paper.

Technical Note: Field-Scale Surface Soil Moisture Patterns and Their Relationship to Topographic Indices

Understanding variability patterns in soil moisture is critical for determining an optimal sampling scheme both in space and in time, as well as for determining optimal management zones for agricultural applications that involve moisture status. In this study, distributed near-surface gravimetric soil moisture samples were collected across a 3.3 ha field in central Illinois for ten dates in the summer of 2002, along with dense elevation data. Temporal stability and consistency of the moisture patterns were analyzed in order to determine a suitable grid size for mapping and management, as well as to investigate relationships between moisture patterns and topographic and soil property influences. Variogram analysis of surface moisture data revealed that the geospatial characteristics of the soil moisture patterns are similar from one date to another, which may allow for a single, rather than temporally variable, variogram to describe the spatial structure. For this field, a maximum cell size of 10 m was found to be appropriate for soil moisture studies on most of the sampling occasions. This could indicate an appropriate scale for precision farming operations or for intensive ground sampling. While some areas had consistent behavior with respect to field mean moisture content, no conclusive relationships between the overall patterns in the moisture data and the topographic and soil indices were identified. There were, however, some small but significant correlations between these two sets of data, particularly plan and tangential curvature, and also slopes. In areas of convergent flow, moisture content exhibited a slight tendency to be wetter than average. There also seemed to be a small influence of scale on the relationship between moisture patterns and topographic curvatures.

Atrazine Losses from Corn Fields in the Little Vermilion River Watershed in East Central Illinois

High levels of atrazine in the Little Vermilion River (LVR) and Georgetown Lake, which supplies drinking water for 5,000 residents, have been detected. Each late spring or summer, the level of atrazine has exceeded the USEPA Maximum Contaminant Level (MCL). In this study, atrazine losses from surface runoff and tile flow under different cropping and soil management practices for four years of monitoring were studied. The loss from surface flow was much less than that from tile flow. The major loss of atrazine occurred within three months after application and was generally caused by heavy rainfall. Early application without anything growing in the field or atrazine application some time later than planting caused an increase of atrazine losses during rainfall. This suggests that the greatest use of atrazine was at the beginning of weed growth. Therefore, the best time for atrazine application is just before or at the beginning of weed growth. The amount, timing, and distribution of rainfall and stage of plant growth influence the movement of atrazine.

MODELING FLOW ON A TILE–DRAINED WATERSHED USING A GIS–INTEGRATED DRAINMOD

Many of the most productive agricultural watersheds in Illinois have mild topography (<1%) and poorly drained soils. The primary mechanism for removing excess water in these watersheds is subsurface (tile) drainage systems. Subsurface drainage systems are a rapid conduit for transporting contaminants, especially nitrate–nitrogen. To better understand the hydrologic response of tile–drained watersheds, DRAINMOD was coupled with Arcview to simulate the hydrologic response of a tile–drained watershed. In this modeling approach, a tile–drained watershed is subdivided into uniform cells, and DRAINMOD is run on each cell with inputs based on the individual characteristics of each cell. The result is a distributed parameter model based on the water balance of DRAINMOD that accounts for surface runoff, subsurface tile flow, and stream baseflow. Daily flow was simulated for the Upper Little Vermilion River watershed in east central Illinois from 1992 to 1997. Individual tile systems within the watershed were mapped by analyzing color infrared aerial photographs to determine the location and extent of subsurface drainage systems. The model was able to adequately simulate the hydrologic response of the watershed. Comparing observed flow with the model’s output over the simulation period returned an R2 value of 0.672 and a standard error of 0.465 mm/day.

ESTIMATING DRAIN SPACING OF INCOMPLETE DRAINAGE SYSTEMS

Determination of nitrate loading per unit area from a field to a subsurface drain requires an estimate of the area contributing flow to the drains. In random drainage systems or systems with irregularly spaced drains, the contributing area is often unknown. Presented are the development and application of a method for using drain outflow rates to estimate drain spacing for incomplete drainage systems that in effect drain most of the intended area. An optimization routine is used to determine the drain spacing that minimizes the difference between observed and DRAINMOD-simulated tile outflows. The method was used to determine the effective spacings of tile drains installed in four fields in the Little Vermilion River watershed in east-central Illinois. The drain spacing information estimated the nitrate loading rates for regions within the watershed. Under the prevailing conditions of the watershed, DRAINMOD performance was relatively insensitive to surface storage, the depth of the impermeable layer from the ground surface, diameter of the tile drains, and the lateral hydraulic conductivities in all the soil layers of the soil profile, with the exception of the layer in which the drains were located. Random tile drains in the agricultural fields on Drummer/Flanagan soils in east-central Illinois have an effective region of influence of 100 m. Effective drain spacing was not site specific, whereas hydraulic conductivity was highly variable and site specific.