Question 13.7: Given the following catchment descriptors for the River Warl......
Given the following catchment descriptors for the River Warleggan at Trengoffe, estimate the value of QMED: AREA = 25.30 km², BFIHOST = 0.500, SPRHOST = 35.7, SAAR = 1518 mm, FARL = 0.9728, URBEXT = 0.0013.
From equation (13.9), AE = 1 – 0.015 \ln \left(\frac{AREA}{0.5}\right) = 1 – 0.015 \ln \left(\frac{25.30}{0.5}\right)
AE = 1 – 0.05885
AE = 0.941
From equation (13.10), \text { REHOST } = \text { BFIHOST }+1.30\left(\frac{SPRHOST}{100}\right) – 0.987
REHOST = 0.500+1.30\left(\frac{35.7}{100}\right) – 0.987REHOST = – 0.0229
Now inserting the values into equation (13.8) gives:
= 1.172 × 20.909 × 1.918 × 0.930 × 0.287 × 1.094
= 13.724 m³/s
Actually QMED = 8.561 m³/s so the factorial error = 13.724/8.561 = 1.60, which is just outside the 68% confidence limit of 1.55 QMED_{rural}. Therefore this is one of the 32% of estimates that fall outside the 0.65 – 1.55 QMED_{rural} range.
Note that in this example URBEXT = 0.0013, which is well below the 0.025 limit, since this is basically an undeveloped catchment. However, FARL is less than the unreservoired default value of 1.0. In fact, this is an area of china clay extraction and there are numerous small lakes in the catchment.