The impacts of land cover in critical water resources in the Motueka River catchment was modelled using the Soil and Water Assessment Tool (SWAT) as part of an Integrated Catchment Management (ICM) project.
The Motueka River basin drains an area of 2075km2 providing ~65% of the major freshwater flow into Tasman bay. The catchment area has a complex mixture of geology and land use and as a result water availability is an important issue. Competing interests between in-stream and off-stream users include exotic forestry, sheep and beef farming, limited but increasing dairying, horticulture and a large area conservation estate.
This study used SWAT, a physically based, distributed hydrological model (PDHM) to model the Motueka catchment. The catchment was first split into sub basins and then further divided into hydrologic response units (HRUs) based on soil and land cover types. An HRU is a base spatial unit upon which swat simulates the water balance.
The study adopted a multi-variable and multi-site approach in the calibration and validation of the SWAT model, making use of extensive field measurements throughout the catchment, and subcatchments focusing on a number of hydrological processes (model components). The internal variables used were potential evapotranspiration (PET), annual water yield, daily streamflow, baseflow, and soil moisture including soil and root-zone infiltration. The study was conducted using an 11-year historical flow record (1990–2000); 1990–94 was used for calibration and 1995–2000 for validation.
Flow calibration process in SWAT
Findings
In applying the SWAT model across the Motueka catchment, the hydrological components of the SWAT model, such as PET, water yield, streamflow and baseflow, were calibrated and validated at whole-catchment scale and for six subcatchments of the Motueka catchment. The approach used for calibration and validation resulted in more realistic values across both the hydrological processes and the geographic areas, and highlighted the areas (e.g. upper Motueka) and the hydrological processes (e.g. soil moisture) requiring greater calibration effort. However, the spatial variability of precipitation could be better represented, this contributing significantly to model errors.
SWAT generally predicted the PET, water yield and daily streamflow with reasonable accuracy. The predicted daily streamflow matched the observed values. The predicted soil moisture levels were higher compared with the measurement. About 50% of the extra soil water storage predicted by the model can be ascribed to over prediction of precipitation; the remaining 50% discrepancy was likely to be a result of poor representation of soil properties.
Given the high spatial variability of the precipitation, the integrated calibration and validation process showed that SWAT had an acceptable hydrological performance in the Motueka catchment. This work was considered as a first step to developing a model useful for catchment planning, and much work still needs to be done on model verification.
After the SWAT (Soil and Water Assessment Tool) model was calibrated and validated to historic flow records for the current land use conditions, in a subsequent paper two additional land cover scenarios (a prehistoric land cover and a potential maximum plantation pine cover) were used to evaluate the impacts of land cover change on total water yields, groundwater flow, and quick flow in the Motueka River catchment, New Zealand. SWAT modelled results indicated that;
- Total water yield under both prehistoric and potential scenarios decreased by around 5% relative to current conditions.
- The maximum potential commercial forestry (pine) scenario still results in higher water yields than the estimated prehistoric flows, although considerably lower flows than the current land use.
- The predicted minimum 7-day low flows reduced by an average of about 20% and 15%, under prehistoric and maximum potential pine scenarios respectively.
Associated Models
Soil and Water Assessment Tool (SWAT)
Soil and Water Assessment Tool (SWAT) is a watershed scale model originally developed by Dr Jeff Arnold for the USDA Agricultural Research Services.
References
Cao, W.Z.; Davie, T.; Fenemor, A.; Bowden, W.B. (2007). Reply to comment on Cao W, Bowden BW, Davie T, Fenemor A. 2006. 'Multi-variable and multi-site calibration and validation of SWAT in a large mountainous catchment with high spatial variability.' Hydrological Processes 20(5): 1057-1073. Hydrological Processes 21(23): 3229-3230.
Cao, W., Bowden, W. B., Davie, T., & Fenemor, A. (2009). Modelling impacts of land cover change on critical water resources in the Motueka River catchment, New Zealand. Water Resources Management, 23(1), 137-151.
http://icm.landcareresearch.co.nz/knowledgebase/publications/public/wenzhi_land_cover.pdf