2. Data Sources
2.1 Watershed Derivation
From the NIWA 30m digital elevation model (which encompasses the whole of NZ), the catchment boundaries upstream of each of the 73 EW sampling points were identified (by NIWA) using a GIS. These boundaries were then passed to EW and overlain on GIS maps of land use, livestock, soil drainage, rainfall, and land management to provide summary statistics (eg. the percent of catchment area by land use). The data provided by this approach gave no indication of the spatial patterns within watersheds. For example, if a watershed was partially forested, it was not known how many stands of trees there were, or their location within the catchment. The field survey identified inaccuracies associated with the derived watershed boundaries. These were attributed to the relatively coarse resolution (30 m) of the DEM.
The watersheds ranged from 2 to 2270 km2 in size and covered c. 50% of the Waikato region (Figure 1). They encompass a range of land uses, soil types, and potential sources of faecal contamination.
Figure 1. Sampling locations (obtained from the Environment Waikato’s Regional River Monitoring Programme, copyright Environment Waikato) and their watersheds, within the Waikato region. The colours are used only to illustrate the extent of each watershed.
2.2 Slope Angle
The digital elevation model (section 2.1) was used to derive the dominant slope angle in each watershed. This was achieved by classifying the slope angle in each 30 ´ 30m cell into one of 7 ranges, with the midpoint of the most frequently occurring range in a catchment being the dominant slope. Additionally, the percentage of steep slopes (>15° ) within each watershed was derived.
2.3 Land Use and Stock Density
Land cover data for each of the 73 watersheds was supplied by EW from the 1996 Land Cover database derived by Terralink New Zealand Ltd. Reference to the map of land cover for the whole region (Figure 2) illustrates the predominance (56%) of pastoral farming. Large areas of plantation forest and indigenous vegetation are also evident, particularly on the Coromandel peninsula, the western fringes, and the south east of the region. Sheep and beef cattle are farmed on the steeper hillslopes, but dairying dominates and is most intensive in the centre of the region near Hamilton (Figure 3). The numbers of beef cattle, dairy cattle, sheep, and deer in each watershed were provided by EW. This data was derived from an agricultural database (Agribase) under licence to AgriQuality New Zealand Ltd. Animal numbers were converted to stock units (Fleming and Burtt 1991) to account for variation in the supply of faecal material with animal type. This approach could not, however, account for differences in the content of E.coli (per unit weight of faecal material) between different animal types. The stock records provided were based upon data collected in 2001, whereas water quality was monitored in 1998-2001 and land use surveyed in 1996-97.
2.4 Soil Drainage
The soil drainage characteristics of each watershed (Figure 4) were provided by EW and derived from the New Zealand soil classification database, Landcare Research New Zealand Ltd. (Hewitt 1993). Within this dataset, soil drainage is classified into 1 of 5 categories, ranging from very poorly drained (Class 1) to well drained (Class 5).
2.5 Rainfall
The distribution of mean annual rainfall across the region was supplied by NIWA – Wellington in a grid format. This data was derived by interpolating a surface of point measurements of annual rainfall, a process that also accounted for the influence of altitude upon rainfall. From the resulting grid, mean annual rainfall was derived for each watershed. The region has an average annual rainfall of 1,250 mm, but this ranges from in excess of 3,000 mm in the Coromandel peninsula, and parts of the south-west of the region, to less than 1,200 mm in the lower Waikato lowlands, the Hauraki plains, Taupo and the Reporoa valley.
Figure 2. Land Cover for the Waikato region, sourced from the Waikato State of the Environment report 1988. Copyright Environment Waikato.
Figure 3. Cattle density and median E.coli across the Waikato region, sourced from the Waikato State of the Environment report 1988. Copyright Environment Waikato.
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