WIND EROSION RATES ON TERRACES IN THE MACKENZIE BASIN

The extent of soil erosion was estimated at six transects on terraces in the Mackenzie Basin using the Cs-137 technique to quantitatively es timate erosion since 1953. Mean Cs-137 areal activity over all sites w as 3.51+/-9 Bq/m(2), compared with an input value of 422+/-63 Bq/m(2), and indicated a mean soil loss of 2.2 cm. Cs-137 areal activities exc eeded the input value at few sites; most of the sites that did exceed the input value were tussock pedestals or well vegetated areas. There was a good correlation between decreasing vegetation cover and decreas ing mean Cs-137 areal activity for Mackenzie soils, but not for Pukaki soils. However, there were strong contrasts in Cs-137 levels between bare sites (268+/-17 Bq/m(2)) and vegetated sites (418+/-15 Bq/m(2)) f or Pukaki soils. Mean loss of Cs-137 from bare ground was 36%, equival ent to a soil loss of 3.9 cm. By contrast, vegetated sites showed no s oil loss. The mean Cs-137 areal activity for all pedestals was slightl y greater (457+/-25 Bq/m(2)) than the input value, while non-pedestall ed sites had a mean of 326+/-9 Bq/m(2) (an average loss of 2.8 cm of s oil). This suggested that vegetated areas, including pedestals, were s table or gaining soil, while bare deflated sites were losing soil. Top soil depths tended to show similar trends to Cs-137 areal activity, wi th thinner topsoils occurring where vegetation cover was poorer. Pedes tals had topsoils that were on average 4 cm deeper than non-pedestalle d sites. The soil losses estimated using Cs-137, contrasts in topsoil depth between pedestals and non-pedestalled sites, and the trend for t opsoil depth to decrease as vegetation cover decreased suggest erosion al losses of a large proportion of the A horizon. The contrasts in top soil depth are greater than the erosional losses measured using Cs-137 , suggesting considerable erosion before 1953.