Using Caecium-137, Lead-210 and particle size fractions to model spatial soil erosion rates in the forest-savannah transition ecology of the Volta Basin of Ghana

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Sustainable land management practices are limited by immediate access to long-term spatial data on erosion rates by all processes. Compared to the current erosion assessment methods of erosion plots, survey of erosion features and modelling, the erosion tracer technique has been found to overcome the above limitations. The objectives of the study were to assess the long-term soil erosion rates under different land uses by using 137Cs, 210pb and particle size fractions (PSFs) as tracers, evaluate the performance of some of the existing calibration models, establish the relationships between the different tracers to facilitate the generation of uniform values irrespective of the tracer used and finally, and to determine the spatial distribution of the tracer-derived erosion rates and their implications on some parameters of topsoil quality. The study was at GLOWA-Volta experimental site on longitude 001° 16’ W and latitude 07° 19 - 20’ N within the Volta Basin of Ghana. Soil and land-use data were collected along grid points for spatial analysis. The data included caesium-137, lead-210, infiltration rate, hydraulic conductivity, particle size distribution, organic carbon, exchangeable potassium, available phosphorus and rainfall. The calibration models were the Proportional (CsPM), Mass Balance 1 (CsMB1), Mass balance 2 (CsMB2) and Diffusion and Migration (CsDM) models for 137Cs, Mass Balance (PbMB) and Diffusion and Migration (PbDM) models for 210Pb, clay ratio (CR), sand ratio (SR) and NUSLE (soil erodibility from the Universal Soil Loss Equation (USLE) nomograph) for particle size fractions. The study determined the reference values of the tracers: 668 ± 90 Bq m-2 and 12680 ± 1700 for 137Cs and 210Pb, respectively, SR was 0.99 ± 0.13, CR (4.85 ± 1.89) and NUSLE (0.58 ± 0.07). CsPM, CsMB1 and NUSLE predicted very low erosion rates typical of fallow lands (S 1.0 Mg ha-1yr-1); PbDM, SR, and CR for moderate erosion rates from a land rotation system ( 5.0 Mg ha-1y-1) and CsMB2 and PbMB for high erosion rates from intensively cultivated fields (: 5.0 Mg h-1y-1). Variance reduction tools and Morgan’s ratio were used to determine the performance of the models. The study established that the risk associated with predicting erosion rates was in the order: CsPM (CV=85 %) <CsDM (CV89 %) <CsMB1 (CV=95 %) <CsMB2 (CV=105 %) for 137Cs models; PbDM (CV=85 %) <PbMB (CV92 %) for 210Pb models; and NUSLE (CV=53 %) <SR (CV=63 %) for PSFs. The sensitivity analyses suggested that losses in 137Cs and 210Pb with respect to the reference values were unstable beyond 40 %; however, SR. CR and NUSLE were stable beyond 50 % loss. Morgan’s performance ratio showed that the erosion rates by CsDM, PbDM, NUSLE, SR and CR were reliable (0.5 - 2.0). The linear and robust linear regressions used to establish inter- convertibility of the tracers suggested the scaling of point values with respect to the reference to obtain a high relationship among the tracers. Kriging was used to determine the spatial distribution of the erosion tracers and some parameters of soil quality. The results suggest that 137Cs and 210Pb erosion rates reflect the contribution of erosion to soil compaction while the movement of available phosphorus and exchangeable potassium were linked to 137Cs and 210Pb distributions, respectively. NUSLE and SR erodibilities were associated with the loss of organic carbon content of the soil. The study recommends the use of caesium-137, lead-210, Sr and NUSLE for monitoring the long-term spatial erosion rates in the Forest—savannah transition ecology of the Volta Basin for sustainable land use planning.
A thesis submitted to the Department of Crop Science, College of Agriculture and Natural Resources, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, 2006