Ingrid Wesström

IDENTIFICATION OF HYDROLOGICAL FACTORS CONTROLLING PHOSPHORUS CONCENTRATION IN DRAINAGE WATER IN SANDY SOILS

ABSTRACT The relationship between total phosphorus (TP) and molybdate-reactive phosphorus (MRP) concentrations in subsurface drainage waters in the hydrological conditions prevailing during autumn and spring flow events was statistically analysed using multiple linear regression analysis. Data on hydrological conditions in three drainage experimental plots in a loamy sand in south-east Sweden complemented with DRAINMOD-predicted data were used as independent variables. Regression models explained at least 80% of the variation in TP and MRP concentrations in drain outflow, based on adjusted coefficient of determination ( R 2adj ) calculations. DRAINMOD-predicted cumulative infiltration ( INFIL cum ) was identified as the most important hydrological condition controlling TP and MRP concentrations in drain outflow in three autumn events and in two out of three spring events. This suggests that the first infiltrating water found more soluble P forms available for transport, after which TP and MRP concentration in drainage outflows gradually decreased during the flow events.

Method for in situ measurements of water, sediment and phosphorous transport in the upper soil profile

Abstract Research on transport processes involved in P losses by subsurface flow, including artificial drainage, is becoming increasingly important. An improved understanding of the processes involved in sediment detachment and P mobility in the soil profile is needed in order to develop proper management strategies for P control. The objective of this article is to present the development of a field lysimeter technique, and a complementary laboratory pinhole test on undisturbed soil samples, that can be used to assess rates and mechanisms involved in water, sediment and phosphorus transport in the upper soil profile at different rainfall intensities. The study, performed on a silty clay, consisted of three parts: (1) pre-test of a new field lysimeter approach with the aim to observe water flow patterns and difficulties related to preparation and set-up, (2) evaluation of the improved field lysimeter methodology, which consisted of a rain simulator above ground, the soil profile through which the applied water percolated, and a collection tray at 40–50 cm depth from which the drain water and sediment was sampled, (3) evaluation and development of the pinhole test for assessing soil resistance to internal erosion. The pre-tests gave promising results and the improved field lysimeter showed interesting temporal responses, at two consecutive rain simulation intensities, in outflow rates of water, bromide, lithium, total P and dissolved P. The pinhole test was run on undisturbed samples with three different water contents and at three different applied positive pressure heads and showed fast peaks in turbidity following start of each run. Combining measurements from the in situ field lysimeter and pinhole approaches presented in this article has the potential to be valuable in detecting critical parameters that control the processes leading to subsurface leaching of P to deeper soil layers.

Influence of higher rain intensities on phosphorus movements in the upper half meter of macroporous clay soil

In climate change scenarios, the frequency of high-intensity rain events in Sweden is assumed to increase. In a plot experiment at Ultuna, Uppsala, the influence of rain intensities on phosphorus (P) transport in the uppermost 0.5 m of a clay soil was studied at 16 locations. A rain simulator, 0.5 × 0.5 m and mounted 1 m above the soil surface, was used to simulate 85–500 min rain sequences causing small (4–9 mm h−1) and large (22–28 mm h−1 and one extreme at 37 mm h−1) steady water fluxes (intensity) in the underlying soil profile. Water percolated to a zero-tension collector tray at 0.5 m depth where drain water and its sediment load was sampled at discrete time intervals. The total P (TP) mass flux ranged, at low intensity, between 12–92 μg m−2 min−1 (average 28.1 μg m−2 min−1) and, at high intensity, between 83–375 μg m−2 min−1 (average 168.5 μg m−2 min−1) and 648 μg m−2 min−1 at the extreme intensity. The soluble reactive (inorganic) P (SRP) mass flux ranged, at low intensity, between 1–65 μg m−2 min−1 (average 10.0 μg m−2 min−1) and, at high intensity, between 6–205 μg m−2 min−1 (average 47.9 μg m−2 min−1) and 495 μg m−2 min−1 at the extreme intensity. Thus, in the intensity range 4–28 mm h−1, TP and SRP increased, on average, by approximately 12% (μg m−2 min−1) per unit increase in intensity (mm h−1). The results of this study demonstrate increased sediment and P loss/mobility for clay soil under increased precipitation intensity predicted under climate change.

Effects of tile drainage repair on nutrient leaching from a field under ordinary cultivation in Sweden

Leaching losses of nitrogen (N), phosphorus (P) and potassium (K) from arable land can be high, with N and P contributing significantly to the eutrophication of lakes and coastal waters. This study examined whether agriculture management and drain repair changed the chemical properties of shallow groundwater and affected nutrient leaching in the field. The hydrology of a subsurface-drained agricultural observation field included in the Swedish water quality monitoring programme was simulated for the period 1976–2006 using the process-based, field-scale model DRAINMOD. On the assumption that the drainage system operated similarly before and after repair, 54% more water was assigned to low-moderate flow events. Measured concentrations of sulphate-sulphur (SO4-S), sodium (Na), chloride (Cl) and potassium (K) were significantly lower in shallow groundwater in the period before drainage system repair (1980–1998) than afterwards (1998–2010). The concentrations were also significantly correlated with the corresponding concentrations in near-simultaneously sampled drain water. A similar connection was not observed for Na and Cl in the period before drain repair. Elevated concentrations of nitrate-nitrogen (NO3-N) were recorded both in shallow groundwater and in drainage water from 1998 to 2010, especially after incorporation of chicken manure into the soil in 1998. Based on simulated discharge (assuming a functioning measuring station throughout), estimated flow-weighted mean NO3-N concentration in drainage water increased from 5.6 mg L−1 (1977–1998) to 15.7 mg L−1 in the period 1998–2000. Simultaneously, mean NO3-N concentration in shallow groundwater increased from 0.2 to 4.0 mg L−1, and then to 4.8 mg L−1 in the period 2000–2012. It was estimated that after drain repair, a greater proportion of infiltrated NO3-N entered the receiving stream directly via the outlet of the tile drainage system close to the fields monitoring station than was the case before repair.

Status assessment of agricultural drainage ditches

Abstract. Poor maintenance, environmental concerns, land use changes, and adaptation to climate change are creating a growing need for better agricultural drainage. The objectives of this study were to identify ditch properties that can be evaluated visually on-site and related soil erosion processes, and to define parameters requiring more intensive study and estimate these using simplified methods. The study included surveys of ditches in various soils using MADRAS (Minnesota Agricultural Ditch Research Assessment for Stability) to classify the status of these ditches. To explain why some ditch sections were in poor condition, additional field and laboratory studies were carried out. Soil samples were taken for analysis of particle size distribution, near saturation shear strength (VJ Tech), and cohesion strength. The data model HEC-RAS was used for simulation of hydraulic forces acting at different flow rates. Digital maps of land use in the catchment area in different years were used to estimate changes in runoff conditions over time. MADRAS proved to be a suitable tool for rapid assessment of stability problems in ditches. HEC-RAS simulations were a good complement to MADRAS in assessing how changes in land use affected the hydraulic load and in highlighting bottlenecks in systems. However, the hydraulic load did not adequately explain the degradation degree in some ditch sections. Measurements of soil shear strength were a good aid to understanding existing degradation. Thus assessment of sensitivity to erosion and bank failure are essential in anticipating the risks of future erosion processes in ditches.

In Situ Method for Measuring Water Fluxes, Sediment, and Phosphorus at High Drip Infiltrometer Intensities in the Upper Half Meter of a Tilled Clay Soil

The first step in evaluating phosphorus (P) loss risks should be to investigate the topsoil, which is generally considered a source of P transport via macropore flow. A procedure is presented for in situ measurement of hydraulic response times, critical water outflow rates, as well as turbidity (T), sediment (SC), and total phosphorus (Ptot) concentrations in outflowing soil water solution from the upper half meter of a clay soil. The method applies to a range of controlled experimental rainfall intensities from a drip infiltrometer, and a zero-tension collection tray located at 0.5 m depth through which percolating water/sediment solution is sampled. Reasonable positive relationships were observed between T, SC, and Ptot versus steady output flow rates (qs). Dependencies were strong between Ptot and each of qs and T, and weaker between Ptot and SC. The methods require further validation and will be further developed in upcoming studies.

A tool for assessing the status of drainage ditches and the need for remedial measures

Poor maintenance of drainage systems has been identified as one of Swedish agricultures weak points, as impaired land drainage can lead to major environmental problems. Drain maintenance work is receiving increasing attention in work conducted within the European Union Water Framework Directive. Moreover, the functionality and maintenance of agricultural ditches will need to be improved to cope with predicted higher pressures on water infrastructure in a future climate. In Sweden, ditch maintenance work is mandatory, but must be carried out with great consideration for the environment. There is currently no simple method available in Sweden for assessing the comprehensive status of open drainage ditches and determining whether remedial measures are needed. This study tested an on-site method, Minnesota Agricultural Ditch Research Assessment for Stability, for evaluating the status of drains and the need for improvements. The study also included an evaluation of physical ditch properties that can be assessed visually on-site and the processes they represent. The results showed that it is viable to evaluate ditch properties such as bank stability, erosion and deposition susceptibility visually on-site. However, more intensive surveys are needed to identify the processes affecting bank stability. For assessing how improvement of drain status affects bank stability and hydraulic properties, Hydrologic Engineering Centres River Analysis System simulations can be a useful tool when complemented with updated cross-section measurements and existing documentation.

Predicted Impacts of Climate Change on Crop Production on Drained Lands in Sweden

We have conducted a simulation study using the hydrologic model, DRAINMOD, and the carbon and nitrogen model, DRAINMOD-N II, to assess the potential impacts of climate change on crop production on drained lands in Sweden. Simulated system include a loamy sand topsoil underlain by a poorly drained clay layer, a winter wheat-sugar beet-spring barley-spring barley crop rotation, and a drainage system composed of subsurface drains (depth=1.0 m, spacing=10, 20 m) managed using conventional and controlled drainage. Two sets of 49-year climate data were used: measured historic climate data for the period 1961-2009 and predicted future climate data for the period 2011-2059 (based on the regional model, RCA3, and the global model, ECHAM4/OPYC3). Climate models predicted an increase in average annual temperature by 1.9oC and a 9% increase in average annual precipitation, both occurring during winter and early spring. In response, DRAINMOD/DRAINMOD-N II predicted a moderate increase in average annual evapotranspiration (approximately 10%) and a slight increase in average annual drainage (less than 4%). Over the 49-years, a 3% reduction in soil organic carbon was predicted because of faster decomposition during warmer winter and spring. The increase in predicted drainage and mineralization of organic nitrogen caused an increase in predicted N drainage losses. The predicted increase in denitrification during the warmer winter and spring improved the performance of controlled drainage for reducing N drainage losses. The model predicted a slight increase in crop yields of winter wheat and spring barley (less than 3%) and 7% reduction in the sugar beet yield.

Storage and Reuse of Drainage Water

The effects of drainage water storage in ponds on nutrient leaching and water resource management were examined in a three-year (2006-2008) field experiment in a 163 km2 study area in southern Sweden. The land use in the area is mainly intensive agriculture and approximately 2.5 million m3 of groundwater are used for irrigation every summer. In 2004, 27 small water storage ponds were constructed in the area. The total storage capacity of these ponds is 355 000 m3 and if they were to be refilled e.g. 1.5 times per season, the groundwater use for irrigation could be decreased by 20%. This study examined the effects of the ponds on nutrient transport and water resource management and developed an index for risk assessment of drainage water quality. Weather parameters and changes in water storage were recorded in the field and samples of water entering and leaving the ponds were collected. Analyses of the water revealed that the ponds acted as a trap for transported nitrogen and phosphorus within the catchment. Digital data on land use, soil type, drainage network and slope gradients were used to identify watershed boundaries and to evaluate the impact of watershed properties on water quality. The potential non-point pollution indicator method (PNPPI) developed for assessing catchment potential as a contributor of nitrogen and phosphorus leaching proved useful. However, the temporal variability was not fully considered and a procedure for including point sources of pollution should be added.