Spatial scales of ¹³⁷Cs-derived soil flux by wind in a 25 km² arable area of eastern England

Abstract

The area effected by wind erosion in England is estimated to be small, but the magnitude of the problem within this area is unknown. Direct measurement of the process is difficult because of very high spatial and temporal variability, selectivity and its slow, insidious nature. The artificial radionuclide caesium-137 (¹³⁷Cs), offers an alternative method. It was used here to estimate the net (ca. 35 year) soil flux (erosion and deposition) in a 5×5 km area (ca. 19 km² area sampled) of East Anglia. It is the first study in the UK to investigate the continuous spatial variation of ¹³⁷Cs over an area of such a size and one of only a few in the world to focus specifically on the redistribution of soil by wind. A two-stage, nested sampling frame captured approximately 50% of the variation that occurred between and within fields. A total of 148 samples were taken and their analysis was used to produce a variogram of the spatial variation. A spherical model was fitted to the experimental variogram using a weighted least-squares procedure. Simulations of sampling configurations on a regular grid did not provide a practical improvement to the nested sampling frame within the specified tolerance. Instead, the parameters were used in ordinary kriging to map ¹³⁷Cs every 50 m at unsampled locations across the area. A tentative value for the newly established ¹³⁷Cs reference inventory for the region was 2068±130 Bq m⁻². Owens' mass–balance model of the relationship between ¹³⁷Cs movement and soil redistribution was modified to the spatially distributed situation and also so that it could account better for the factors that control wind erosion and deposition (Towards improved interpretation of caesium-137 measurements in soil erosion studies. Unpublished PhD thesis, Exter University). A calibration relationship for each field was used to calculate the net soil flux at every 50 m. The sample region was found to approximately balance with a net soil loss of 0.6 t ha⁻¹ year⁻¹; the range was −32.6 to +37.5 t ha⁻¹ year⁻¹. Soil from the high-loss fields was accumulating in field boundaries. Despite little of the material leaving the region, the effect of soil nutrient loss on the fields may be considerable. It appears that wind erosion may have as great or greater impact in the areas where it is active than does water erosion. Net soil flux was inferred to be the result of wind erosion, but soil loss on harvested crops (primarily sugar beet), and perhaps losses during tillage (pulverizing erosion) may be other contributors, and this requires future study. Moreover, the quantity of soil lost may not be as important as its quality. Future work is therefore also required to investigate the mass–balance of soil nutrient enrichment and depletion as a consequence of soil erosion and fertiliser application.

Introduction

Little is known about the extent and magnitude of wind erosion in Europe. In Britain, considerably more work has been undertaken to measure and model the processes and impacts of water erosion than those of wind erosion, despite intuitively reasonable estimates that wind erosion is serious on sandy soils (MAFF, 1997).

The direct measurement of soil flux (erosion and deposition) by erosion (as of that by water) is difficult because of the high spatial and temporal variability of the processes. The use of the artificial radionuclide caesium-137 (¹³⁷Cs), which can be used to estimate soil loss and gain over the last ca. 35 years, overcomes some of these problems. It has been used successfully in many parts of the world to estimate net soil redistribution by the combined effect of wind, water and more recently by tillage erosion Walling and Quine, 1993, Quine et al., 1996. Despite some limitations (Chappell, 1999) and some uncertainties in cultivated situations (Bremer et al., 1995), its use is now well established. In Britain, soil flux rates established in this way range from 0.1 to 50 t ha⁻¹ year⁻¹ (Owens and Walling, 1996). There have been few applications of ¹³⁷Cs for estimating erosion by wind alone. Sutherland and de Jong (1990), Basher et al. (1995) and Harper and Gilkes (1995) inferred that losses from level cultivated fields were due dominantly to wind erosion, and Quine and Walling (1991) made a similar inference for fields in the English Midlands. Chappell et al. (1997) used ¹³⁷Cs to show that a considerable component of the net soil flux in part of Niger was due to wind erosion, and Chappell et al. (1998b) and Warren et al. (2003) considered the implications of these losses for the cultivation system in the same area. Chappell et al. (1998a) further quantified the rate of dust accumulation in this region. Gillieson et al. (1996) compared potential wind erosion rates with ¹³⁷Cs-derived rates using samples of a large area in Australia.

The ¹³⁷Cs technique for estimating net soil flux is commonly based on establishing the local amount of ¹³⁷Cs deposited from atmospheric weapons testing, mainly during the 1960s (Ritchie and McHenry, 1990). This ‘local reference ¹³⁷Cs inventory’ should come from a site that has been undisturbed by erosion or deposition. The rate of net soil redistribution is based on the relationship between the amount of ¹³⁷Cs at the reference site and the amounts in cultivated fields. When sufficient information on the fate of ¹³⁷Cs exists for a site, its relationship with erosion or deposition may be represented by a mass–balance model Walling and Quine, 1990, Owens, 1994, He and Walling, 1997.

The ¹³⁷Cs technique is commonly applied to small areas, and seldom to areas bigger than single fields (cf. Walling and Quine, 1993, Sutherland and de Jong, 1990, Loughran et al., 1993). Samples are usually taken from the nodes of a regular grid, because little is known about the spatial scale of variation in soil erosion processes. Sampling grids are usually coarse, because the measurement of ¹³⁷Cs is time-consuming, but if the grid spacing is too large, the samples may not represent the true variation, and could provide a biased estimate. Use of a fine grid, on the other hand, might create considerable redundancy. Many of these problems are overcome with a nested, stratified approach Chappell et al., 1997, Chappell, 1998, which can be combined with the variogram to summarise the amount of variation between different sample separation distances de Roo, 1991, Chappell and Oliver, 1997. This summary tool is effective for elucidating the scale of processes controlling land surface formation (Chappell et al., 1997) and is essential for estimation at unsampled locations.

The aim here is to map ¹³⁷Cs-derived estimates of net (ca. 35 year) soil flux (erosion and deposition) by wind over a large (5×5 km) highly susceptible area of arable land in Eastern England and to quantify the magnitude of wind erosion over several scales (a) within fields, (b) between fields (farms) and (c) across the region as a whole. The aim has four objectives, to: (1) develop a strategy that was efficient for sampling the spatial variation in ¹³⁷Cs with only 148 samples; (2) model the spatial variation in ¹³⁷Cs with sufficient reliability to map ¹³⁷Cs over the study area; (3) establish the reference ¹³⁷Cs inventory for this region of UK where ¹³⁷Cs sampling had previously not been conducted; (4) model the relationship between ¹³⁷Cs movement and soil redistribution by wind. The spatial variation of wind erosion and deposition processes are considered and the implications of the extent and magnitude of soil flux by wind is discussed. Some implications for future large-area assessment of soil flux rates are considered in the light of the findings.