Pasture Catchment Research Articles

Sources and sinks of nutrients in a New Zealand hill pasture catchment I. Stormflow generation

AbstractThe processes of stormflow generation were studied in a hill pasture catchment near Hamilton, New Zealand. Although rainfall was relatively evenly distributed throughout the year, stormflow was highly seasonal and over 65 per cent occurred during the winter. Three main processes contributing to stormflow were identified which could be related to soil type and physiographic position. On gleyed soils derived from rhyolitic colluvium, saturation overland flow was the dominant process. Hydrographs from ‘Whipkey’ throughflow troughs also indicated that there was a subsurface response (saturated wedge) from this soil type. On steeper convex slopes, more permeable soils were derived from weathered greywacke. The presence of ephemeral springs on the hillslopes and direct observation during storm events indicated that storm runoff was generated as return flow from this soil.It was noted that nitrate concentrations from subsurface sources were 5–10 times higher than surface runoff. This difference in concentration was utilized in a chemical mixing equation which partitioned stormflow sources. This was compared with the stormflow predicted from rain falling on to saturated areas. There was good agreement between the two models for winter‐spring events with respect to the volumes of surface runoff predicted, however the saturated areas model underestimated total stormflow.The results of the study are briefly discussed in terms of the potential for water quality management.

Sediment, phosphorus, and nitrogen in channelised surface run‐off from a New Zealand pastoral catchment

Abstract Surface run‐off derived from 94 m (20°) and 230 m (15°) long hillslopes in a pastoral catchment was collected along a 15.8 m total intercepting face between 1 July 1983 and 31 March 1985; run‐off subsamples were analysed chemically. Surface run‐off occurred on 49 occasions, these “events” ranging from 0.2 to 115 m3. About 50% of the total run‐off occurred in six events. In 20 months the eight individual 1.5–2.4 m long collectors intercepted 4–333 m3 of water. This uneven spatial run‐off pattern, the measured flow rates, and total event volumes indicated that extensive areas on both hillslopes produced run‐off, and that extreme channelisation occurred during movement downslope. Mean flow weighted run‐off concentrations were for total phosphorus (TP) 1053 mg/m3, total dissolved phosphorus (TDP) 300 mg/m3, Kjeldahl nitrogen (TKN) 5898 mg/m3, dissolved Kjeldahl nitrogen (DKN) 1413 mg/m3, NO3‐N 69 mg/m3, suspended sediments (SS) 472 g/m3, and volatile suspended sediment (VSS) 115 g/m3. Estimated catchment surface run‐off yields in 1984 were: TP 0.75 kg/ha, TDP 0.15 kg/ha, TKN 4.43 kg/ha, DKN 0.74 kg/ha, NO3‐N 0.03 kg/ha, SS 374 kg/ha, and VSS 90 kg/ha. Virtually all phosphorus and nitrogen was transported downslope in winter and spring. Controlling these run‐off nutrient inputs to the catchment stream is briefly discussed.

Low flow water chemistry in forested and pasture catchments, Mawheraiti River, Westland

Abstract Water samples taken at 19 locations in the Mawheraiti River catchment at weekly intervals during 1979–80 were analysed for sodium, magnesium, potassium, calcium, phosphate, nitrate, and ammonium ion concentrations and for electrical conductivity. Seasonal discharge effects were apparent, and lithology and land management practice also influenced solute concentrations. Solute concentrations were generally very low; nitrate and soluble phosphate were rarely greater than 0.05 mg.L‐1 and ammonium was rarely greater than 0.01 mg.L‐1. The 4 major cations (Na, Mg, K, and Ca) usually summed to less than 6 mg.L‐1 much of which was supplied by precipitation. Forest management (clearfelling and slash‐burning) caused significant increases in solute concentrations, but concentrations declined rapidly during succeeding months and approached pretreatment levels after 2–3 years. The higher concentrations associated with forest management in small experimental catchments were rapidly diluted downstream; together with the low natural solute concentrations this suggests that harmful downstream effects of management practices are unlikely under low flow conditions.

Effect of aerial topdressing with superphosphate on the loss of phosphate from a pasture catchment

Abstract The aerial application of single superpho.sphate (30 kg P/ha) to a pasture catchment in June 1976 resulted in an immediate increase in particulate P (PP) and dissolved inorganic P (DIP) co.ncentratio.ns in a stream under low-flow conditions; maximum values of 1.56 and 9.83 mg/I, respectively, were attained 5 h after application. The increased concentrations were not sustained, returning to the pre-fertiliser application concentrations (0.01 and 0.03 mg/l for PP and DIP, respectively) 24 h after application. The data indicated that 74% of the fertiliser P falling directly into the stream channel was transported in stream flow in the 24 h of low flow after application. The mean concentrations of PP and DIP were approximately 30 times higher immediately after fertiliser P application but these decreased exponentially with time. If it is assumed that the mean concentrations of P forms in flow events would have remained constant if no fertiliser P had been applied, the increased transport of total P, ...

Evapotranspiration and soil moisture in upland grass catchments in the eastern Pennines

Simple mathematical models are used to investigate the mean moisture states and the actual evapotranspiration from two small pasture catchments in the eastern Pennines, U.K. Both precipitation and runoff were measured, and potential evapotranspiration calculated by Penman's method, so the only unknowns were actual evapotranspiration and soil moisture, and these were estimated using a two-stage soil moisture model. It was found that the catchment covered by typical Pennine rough grazing had an available soil moisture content of about 50 mm and an albedo of 0.18, both somewhat lower than the values for improved pasture. The rough grazing which was discovered to have a rather extreme hydrological regime as compared with improved pasture or normal agriculture crops and soils, became quickly saturated after rain and rapidly dry in summer. It was found on both rough grazing and improved pasture with light or moderate rainfall amounts that the amount of runoff was controlled by the soil-moisture state and not by the infiltration capacity of the soil. With heavy rainfalls considerable runoff occurred while the soil at depth remained moderately dry.

A model for rainfall routing during initial abstraction

More reliable estimation of runoff requires the development of a model to simulate rainfall disposition prior to runoff production on rural catchments. A mathematical model, based on the moisture storage capacities of the upper and subsoil zones of a catchment was developed and its general form was confirmed by comparison of predicted values with recorded data on initial abstraction from four pastoral catchments on texture-contrast soils with heavy clay subsoils at the Parwan Experimental Area, near Bacchus Marsh, Victoria. The modifications necessary for extending this model to ungauged areas were elaborated and changes in the elements of the model following pasture improvement were discussed.

The evapotranspiration component of a pastoral experimental catchment

The amount of evapotranspiration occurring from a pastoral catchment was determined by an assessment of the hydrologic balance within the catchment. A measure of the variable for changes in water content was achieved by the evaluation of soil store deficits antecedent to runoff events. Thus, the intervals of time for the determination of the amounts of evapotranspiration were confined to periods between runoff events. For those intervals when the index of topsoil moisture stress did not exceed a pF value of 3.5, a ratio of one existed between measured evapotranspiration and the index of potential evapotranspiration. With the use of this ratio in conjunction with the soil moisture record, variations in rates of evapotranspiration were presented for the years 1963 to 1966. Such data indicated that the prevailing land-use was inefficient in its consumption of water.