The 1 3 7Cs technique has provided the first quantitative, medium-term data on rates of soil redistribution by surface erosion on both cropland and rangeland in New Zealand. Use of the technique has demonstrated: high rates of soil redistribution by water erosion at two cropland sites under intensive veg e t a ble production; a slow rate of net loss of soil by wind erosion associated with arable fa rming; a strong association between vegetation depletion and wind erosion on grazed rangeland. Research has also provided data on natural short-range variability of 1 3 7Cs in uneroded soils, and a technique for independently estimating 1 3 7Cs reference values from rainfall. The greatest research need remains the development of robust, accurate calibration procedures for conve rting 1 3 7Cs measurements to rates of erosion. Key wo rd s : Caesium 137. Erosion rates. Soil redistribution. Water erosion. Wind erosion. Cropland. Rangeland. Reference va l u e . I N T RO D U C T I O N S u r face erosion by wind and water has been widely r e p o rted on cropland and rangeland in New Zealand (e.g., Eyles, 1983), yet little quantitative information is ava i l able on surface erosion rates. As a result, indirect assessment of surface erosion using the radionuclide tracer caesium-137 (1 3 7Cs) has been used to estimate mediumt e rm surface erosion rates at a number of sites throughout N ew Zealand. The major advantages of the 137Cs technique are: rates of erosion are medium-term estimates (net erosion since the first appearance of 137Cs in the environment); it can be used on both cultivated and uncultivated soils; only a single set of measurements need be made; both erosion and deposition can be measured so the net effect of erosion and/or tillage can be assessed; it has the potential to be used as an environmental indicator to monitor rates of surface erosion in the future. However, the technique requires a number of assumptions about the environmental behaviour of 137Cs, and the relationship between 137Cs loss and soil loss. These are reviewed in detail by Loughran et al. (1990), Ritchie and McHenry (1990), Walling and Quine (1990, 1993) and Basher et al. (1995). Use of 1 3 7Cs has contributed signifi c a n t ly to understanding rates of surface erosion in New Zealand. Emphasis has been on application of the technique to prov i d e field-based data on erosion rates, with limited research to f u rther develop the technique. This paper summarises recent research applying the technique to assessment of water and wind erosion rates on cropland and rangeland, and on development of techniques for estimating reference values and natural spatial variability of 1 3 7Cs. A P P L I C ATIONS OF 1 3 7CS TO SURFACE ERO S I O N A S S E S S M E N T Rates of surface erosion by wind and water on cropland and rangeland have been estimated from va r i a t i o n in 1 3 7Cs areal activity relative to the reference va l u e . Measurement of 1 3 7Cs areal activity has been based on samples taken with small diameter cores (28 or 39 cm2 area), with some taken from a larger area (1300 cm2) using a scraper frame to provide information on the ve rtical distribution of 1 3 7Cs. Each sample of <2 mm airdried soil was analysed for 1 3 7Cs activity at the National Radiation Laboratory, Christchurch by high resolution gamma spectroscopy using HpGe detectors, as described by Basher et al. (1995). The precision of 1 3 7Cs activity measurement was ±5-10%. Erosion rates have gen e r a l ly been calculated by the linear proportional model for cultivated soils and the profile distribution model for u n c u l t ivated soils (see Walling and Quine, 1990, 1993; Basher et al., 1995; Basher and Webb, 1997), ex c e p t where otherwise indicated. Locations referred to in the t ext are shown in Fig. 1.

Surface erosion assessment using 137Cs: examples from Nez Zealand

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