A study of phosphorus exports from two Radiata pine forested catchments was carried out between August 1984 and November 1985. The objective of the study was to determine the impact of a phosphatic fertilizer application on levels of phosphorus exported from one catchment. The study method employed was a paired catchment experiment. Two adjacent catchments with similar hydrologic and physiographical features were chosen as the catchment pair. one catchment (Sherwood south; area 31ha) was treated with 240kg/ha of fertilizer by hand and 40kg/ha by aerial broadcast. The other catchment (Sherwood North; area 27ha) was left undisturbed as a control. Results indicated that phosphorus exports from the fertilized catchment retained constant relative to exports from the unfertilized catchment. The lack of response of phosphorus export levels to the fertilizer addition was attributed to the hydrologic conditions existing during the study period and to the method of fertilizer application. An extended dry period following the application to the south catchment ensured that the fertilizer had assimilated into less mobile forms before the first streamflow event (five months later). Also the bulk of the fertilizer was applied by hand, so that areas susceptible to conditions necessary for phosphorus export (overland flow generation areas) were avoided; thereby minimising potential exports. The study also examined two methods of identifying overland flow generation areas. The first method used topographic characteristics of the catchment to predict areas of relative wetness. The second method used air photo interpretation techniques to map pine tree growth Which was then related to shallow well water table measurements. The importance of these areas in influencing pollutant exports is discussed and management strategies are described to reduce the impact of fertilizer additions on phosphorus export levels. The study also addressed the influence of stream discharge on Phosphorus loads exported. The greatest proportion of loads exported from each catchment occurred during storm events. In the south catchment ‘72% of total phosphorus exports occured during storm events (only 9.51 of the total study period). Phosphorus versus stream discharge regressions were used to estimate loads exported from each of the study catchments. These rating curves were in error by up to 300% in some estimations of monthly phosphorus export loads. Such regressions should therefore be used with caution in dynamic systems where hydrologic conditions exhibit such marked variation.
|Date of Award||1986|