High-frequency monitoring of water fluxes and nutrient loads to assess theeffects of controlled drainage on water storage and nutrient transport

摘要

High nitrogen (N) and phosphorus (P) fluxes from upstream agriculturethreaten aquatic ecosystems in surface waters and estuaries, especially inareas characterized by high agricultural N and P inputs and densely drainedcatchments like the Netherlands. Controlled drainage has been recognized asan effective option to optimize soil moisture conditions for agriculture andto reduce unnecessary losses of fresh water and nutrients. This is achievedby introducing control structures with adjustable overflow levels intosubsurface tube drain systems. A small-scale (1?ha) field experiment wasdesigned to investigate the hydrological and chemical changes afterintroducing controlled drainage. Precipitation rates and the response ofwater tables and drain fluxes were measured in the periods before theintroduction of controlled drainage (2007–2008) and after (2009–2011). Forthe N and P concentration measurements, auto-analyzers for continuousrecords were combined with passive samplers for time-averaged concentrationsat individual drain outlets. The experimental setup enabled thequantification of changes in the water and solute balance after introducingcontrolled drainage. The results showed that introducing controlled drainagereduced the drain discharge and increased the groundwater storage in thefield. To achieve this, the overflow levels have to be elevated in earlyspring, before the drain discharge stops due to dryer conditions and fallinggroundwater levels. The groundwater storage in the field would have beenlarger if the water levels in the adjacent ditch werecontrolled as well by an adjustable weir. The N concentrations and loadsincreased, which was largely related to elevated concentrations in one ofthe three monitored tube drains. The P loads via the tube drains reduced dueto the reduction in discharge after introducing controlled drainage.However, this may be counteracted by the higher groundwater levels and thelarger contribution of N- and P-rich shallow groundwater and overland flow tothe surface water.