Water and nutrient budgets at field and regional scale : travel times of drainage water and nutrient loads to surface water

Keywords : water and nutrient budget, travel time of drainage water, dual-porosity concept, agricultural nutrient losses, loads to surface water, field-scale experiments, regional-scale approach.

 

Nitrogen and phosphorus loads to surface waters have caused eutrophication problems in the Netherlands. Currently, surface water quality standards are not met due to loads from point and diffuse sources. Linking agricultural nutrient budget surpluses to observed losses of nutrients to the surface water by drainage requires water and nutrient balances as well as a travel time distribution of drainage water. In this research the travel time of drainage water was estimated by using steady-state flow analysis as well as transient modeling i.e., using a 2D dual-porosity water flow and solute transport code . To analyze and estimate nutrient loads to the surface water, this research covered both field-scale analyses as well as a regional-scale approach .

At the Flevoland field experimental site , arable crops were grown on clay soil. The presence of a non-ripened clay layer in the soil profile appeared to be a key factor in the division of total drainage into tile drainage, drainage through a collecting and draining ditch, and drainage through the regional surface water. Tile drainage in the 1992-1994 period was between 50% and 85% of the total drainage. Nutrient losses by drainage were 55-86 kg ha ^-1 a ^-1 N and less than 0.5 kg ha ^-1 a ^-1 P and measured nutrient loads of drainage water exceeded the nutrient balance surpluses. The travel time distribution was first estimated by using a steady-state approach, and then confirmed by transient modeling, with some deviations, mainly during dry years. The travel time distribution of drainage showed that 50-60% of the local drainage water had a travel time of less than two years. Taking the travel time distribution into account, the observed N-loads of drainage water could be reasonably predicted. The P-surplus was almost fully adsorbed by the clay soil. The Hupsel-Assink field experimental site was located on grassland on a loamy sand. About 70% of total drainage was discharged by tile drains in the 1993-1994 period. N-losses by drainage were 165 kg ha ^-1 a ^-1N and the calculated P-loads were unreliable. About 50-70% of the drainage water had a travel time of less than two years. The travel time distribution was estimated by using a simple steady-state approach, and then confirmed by transient modeling. Using the travel time distribution, the observed drainage N-loads could be reasonably predicted. The P-surplus was almost fully adsorbed by the loamy sand. Both field experiments showed the relevance and importance of a thorough data analysis and of the travel time distribution of drainage water, which is needed to interpret and link agricultural practices to nutrient losses by drainage.

The Regional scale approach estimated annual water flow and solute loads to the regional surface water, taking point sources and groundwater seepage into account. Annual water, salt, nitrogen, and phosphorus budgets were calculated for the Hoge Afdeling region in the Flevopolder area (1988-1999) and the Hupsel brook basin (1985-1994). A regional-scale travel time distribution of drainage water was estimated. Annual averages of nutrient loads to the regional surface water were well predicted. Nutrient losses by drainage from agricultural land and groundwater seepage were important sources in the Hoge Afdeling region, whereas in the Hupsel brook basin, nutrient losses by drainage from agricultural land were the main source. Accurate solute load estimates of the regional surface water based on well-designed surface-water-quality monitoring programs proved beneficial.