Estimating future scenarios for farm-watershed nutrient fluxes using dynamic simulation modelling – Can on-farm BMPs really do the job at the watershed scale?

Mark Rivers1,2, David Weaver1,3, Keith Smettem1, and Peter Davies4.

1. Centre for Ecohydrology, School of Environmental Systems Engineering, University of Western Australia2. Department of Agriculture and Food Western Australia, Waroona District Office, Waroona, Australia

3. Department of Agriculture and Food, Western Australia, 444 Albany Hwy, Albany, Australia4. Centre for Natural Resource Management, Albany Campus, University of Western Australia. Albany, Australia

Contact Details: Mark Rivers

Email: mrivers@agric.wa.gov.au Tel: +61 (08) 9733 7777

Losses of phosphorous (P) and nitrogen (N) from land to water have been accelerated globally over the last 50 years due to intensification of landscape development for agricultural and urban pursuits.

In Australia the problem is exacerbated by the fact that many waterways were historically oligotrophic so excess nutrients have a greater impact on natural aquatic ecosystems than in systems with higher natural nutrient levels.

Also, the inherently low levels of nutrient-use efficiency in agriculture results in more nutrients imported into agricultural watersheds than are exported from them in the form of agricultural produce. This results in nutrient enrichment of the farm-watershed system and ultimately in increased rates of nutrient loss from the farm to the broader watershed.

The work reported here combines measured and surveyed nutrient data at the farm and watershed scales with a watershed-scale dynamic model of nutrient fluxes to assess future nutrient loading scenarios for the Peel-Harvey watershed, a major coastal watershed in Western Australia (figure 1).

RESULTS AND DISCUSSION INTRODUCTION

METHODOLOGYSurvey data was used to determine the present day inputs of P into the watershed. Regional soil P and P retention test data was used to develop nutrient routing algorithms and locally-derived BMP effectiveness values were used to calculate potential P export reductions.

Using this data we developed a simulation model using the “STELLA©” dynamic modelling software package which estimates P transport through major source, sink and flow sectors of the Peel-Harvey watershed (figure 2).

This was then used to track changes in stores and flows of P over a 100 year time horizon to match watershed development and associated nutrient inputs and outputs to the present day (100 years of development). This presented a point of calibration and modelled results were compared to present day monitored estuary water quality. The model predicts an annual P flow to the estuary of 138 tonnes which compares well with the monitored load of 140 tonnes.

The model was then executed for a further 100 years into the future to determine future estuary water quality under both the present watershed management scenario and under a variety of proposed and actual BMP implementation strategies.

The model allows for nutrient storage, assimilation and release from the various components of the watershed environment (soil, runoff, groundwater, stream sediment and estuarine water and sediment) and the present watershed condition is calibrated against available monitored water quality data, survey data of farm P budgets, and soil P data.

Over the course of a 200 year simulation under the current watershed management scenarios, releases of P from the soil and sediment stores will reach their maxima approximately 20 years from the present day and will not reduce from this value (figure 3). That is: the P “storage” components of the watershed are already “leaking” P, and their ability to buffer further P applications will be almost exhausted in 20 more years if current practices continue. From this point onwards, almost an equivalent amount of P will be released into the regional waterways as is presently applied every year – almost 1400 tPpa. The P load target for this estuary is 70 tPpa, with current loads around 140 tPpa.

CONCLUSIONS

If nutrient input rates into the Peel-Harvey Watershed continue at current levels then this, combined with the expected reduction in buffering capacity of the soils and streams, will lead to significant increases in annual nutrient loss rates even if P-retentive BMPS are implemented on a broad scale. This will have major environmental implications for a watershed and associated waterways already under severe stress and questions the sustainability of the current regional land use mosaic.

Figure 1: Research site location, land use map and annual, watershed-scale phosphorus (P) budget. In the P budget diagram, the width of the arrows and the coloured bars within the arrows are proportional to the amount of P they represent. Cattle for beef can be seen as the major contributor to the overall watershed P budget.

Figure 2: Simplified schematic representation of STELLA P-transfer model

BMP-implementation scenarios which were tested included “biological” interventions such as the planting of perennial pasture species and improved management of riparian vegetation, and “chemical” interventions such as use of P-retentive soil amendments and the use of low-solubility P fertilisers. Various combinations of these interventions were tested also (figure 4).

Only under scenarios where all P-retentive BMPs are implemented at a very broad scale will any significant P-loss reductions be made. None of these, however, attain the target estuary load of 70 tPpa.

In order to facilitate easier modifications to test scenarios, the STELLA model was also exported to Forio Simulate© (figure 5). This model remains online at http://forio.com/simulate/mrivers/peel-harvey/run/#p=page0 and is available for testing by any interested parties.

Figure 3: Modelled component P transfers over 200 year simulation for model P store compartments Figure 4: Annual P load to the estuary under various BMP implementation scenarios

Figure 5: Introductory web page for Forio Simulate© P management model

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