Nitrogen Reductions Due to Water Control Structures as Best Management Practices

Delaware’s poorly drained lands have been outfitted with an intensive and extensive artificial drainage system.  More than 200 individual tax ditch organizations exist statewide, managing roughly 2,000 miles of channels.  These ditch systems are often necessary in coastal plain regions in order to create agricultural land suitable for cultivation on the very flat topography.  Ditching drains excess water during wet periods, such as when the farmer must prepare seedbeds in the spring and harvest in the fall.   Since both of these processes require large farm equipment to traverse cropland, it is undesirable to have overly moist soils.   During dry periods, however, ditches do not allow for water conservation, and over drainage often occurs. In addition to increasing total water discharge, these additional conduits often increase pollutant loads to receiving waters since croplands are typically nutrient rich environments.  In coastal plain settings, ditch systems often drain to estuaries, as is the case in Delaware, which are valued as both ecological and economic resources.  Thus, reducing non-point source pollution is necessary in order to preserve these ecosystems.

Water table control through the use of water control structures is an agricultural management practice used in several coastal plain states for water conservation.  These structures simply consist of a flashboard riser at the ditch outlet.  Flashboards can be lowered or removed during wet periods to allow excess water to drain quickly.  Alternatively, flashboards can be added to the riser to slow field drainage during drier periods, which causes the water level in the ditches and under surrounding lands to rise so that the water is available to the crop and not flushed away unused.  Several studies in the North Carolina coastal plain have found that controlled drainage of this type can decrease water outflow between 20% and 40% depending on climatic conditions, soils, and management intensity (Evans et al., 1989; Evans et al., 1996; and Osmond et al., 2002).

Water control structures are also efficient best management practices (BMPs) for water quality, when properly designed and managed.  Nitrogen loads from agricultural fields with water control structures have been found to be 45% less than from fields without this practice, on average (Evans et al., 1989; Evans et al., 1996; and Osmond et al., 2002).  The loading reduction occurs as the result of two processes.  Loads are the product of water discharge and nutrient concentration, often expressed in units of pounds/acre/year.  Water control structures, as mentioned above, reduce the total volume of water flow.  In addition, the nitrogen concentrations in the drainage waters are often reduced.  Since water control structures provide crops with soil water during the dry growing season, more nitrogen can be taken up by crops.  During the non-growing season, water control structures still provide opportunities for nitrogen reduction because denitrification can occur in the surface soils.  Thus, for maximum nitrogen removal, water control structures must be used and managed year-round and not solely used to remove excess water during critical agriculture operation periods.

Phosphorus load reductions of 35% have also been attributed to water control structures in North Carolinian studies (Evans et al., 1989; Evans et al., 1996; and Osmond et al., 2002).

In Delaware, Total Maximum Daily Loads (TMDLs) are being promulgated for impaired water bodies.  In order to achieve the necessary nonpoint source nutrient reductions, the continued use of BMPs on agricultural lands is being promoted.  In order to estimate nutrient reductions due to the BMPs currently in place and the potential reduction from future additional BMPs, a nitrogen reduction efficiency of 33% has been assigned to water control structures.  This is less than the efficiency reported in the North Carolinian studies.  Best professional judgment suggested that since similar studies have not been performed in Delaware watersheds, a conservative estimate should be used until such data was available to confirm the higher efficiency.

Additionally, Delaware managers have chosen not to assign a phosphorus reduction efficiency to water control structures.  The primary reason for this decision is that this practice is most widely used in Sussex County where approximately one third of the soils are high or excessively high in phosphorus.  The effects of this soil characteristic may alter the extent to which water control structures can reduce the phosphorus concentration of drainage waters, and until better information becomes available, a phosphorus reduction efficiency will not be used.

Evans, R.O., J.W. Gilliam, R.W. Skaggs. 1989. Effects of Agricultural Water Table Management on Drainage Water Quality. The Water Resources Research Institute, Report No. 237.

Evans, R.O., J.W. Gilliam, R.W. Skaggs. 1996. Controlled Drainage Management Guidelines for Improving Drainage Water Quality. North Carolina Cooperative Extension Service, Publication Number: AG 443.

Osmond, D.L., J.W. Gilliam, and R.O. Evans. 2002. Riparian Buffers and controlled Drainage to Reduce Agricultural Nonpoint Source Pollution. North Carolina Agricultural Research Service Technical Bulletin 318, North Carolina State University, Raleigh, NC.