Monitoring soil loss and redistribution using 137 Cs



Kachanoski, R. Gary
von Bertoldi, P.

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Agriculture Canada


At a number of spatial scales (plot, field, farm, watershed, region) the amount of past soil loss is an important parameter in estimating the current state of the soil resource. However, even though soil erosion has been identified as the single most significant factor resulting in the degradation of the land base, there have been few actual measurements of the amount of erosion occurring. Soil loss values are not available because of the difficulty and expense in obtaining measurements. However, in recent years there has been increased interest and use of fallout 137Cs from atmospheric testing of thermonuclear devices in the early 1950's and 1960's. The current level of 137Cs in soil relative to the baseline level that would be present if no soil loss had occurred is a good indication of the cumulative soil loss at a site. Thus, it would be possible to easily benchmark all current and future study sites by measuring 137Cs. However, reliable estimates of baseline amounts of 137Cs (if no soil loss had occurred) are needed. Baseline values of 137Cs can vary significantly, but have been correlated to local precipitation patterns. The two major objectives of this project are (1) Construct a map of base-line 137Cs (total deposition, Bq m-2 ) values for south-western Ontario, and (2) Determine the redistribution of 137Cs/soil since 1965 within a watershed typical of Ontario conditions, including deposition within the watershed, export out of the watershed, and loss from the uplands. Construct a map of the redistribution with geographic positioning so that the watershed can be used as a long-term monitoring site for similar studies in the future. A total of 42 sites were selected on the basis of availability of long-term precipitation records and geographic position. At each site an undisturbed, uneroded area was located and two composite soil samples were taken to a depth below the top of the B horizon. The soil samples were dried and analysed for fallout 137Cs using high resolution gamma-spectroscopy. The data indicate that 82% of the variability of the 137Cs values was accounted for by differences between sampling locations, and only 18% of the variability was attributed to within site variability. The 137Cs values ranged from 1861 to 3015 Bq m-2 and averaged 2420 Bq m-2 . Significant spatial trends were observed in the data and the differences were correlated to the precipitation during the major years of deposition. A baseline map was constructed based on the correlation model to precipitation and the meteorological data from 360 Meteorological stations. The Kintore watershed was chosen as the site for objective 2. The site has a significant amount of baseline information already collected and the Univ. of Waterloo's water partitioning study is being carried out within the watershed. In cooperation with the Upper Thames Conservation Authority, land owners within the basin were contacted and agreed to allow access to their land. Sampling was stratified to characterize fields as well as the major landforms within fields. Measurements consisted of 2,840 surface elevation points obtained from a high precision theodolite (total station) and 352 soil sampling locations for determination of 137Cs. The measurements were geopositioned and combined with a detailed 1:5000 soil map into a common digital spatial database for map production. A digital landform classification was completed and further divided into soil loss classes using the 137Cs data. Soil map units were also divided into soil loss classes. A mass balance of 137Cs indicated an average soil loss rate of 4.8 t/ha per yr. However, 5 % of the sub-basin had severe erosion (30-70 t/ha per yr.), and 24% had moderate erosion (15-30 t/ha per yr). Significant deposition occurred in 6% of the study area. The spatial information can be used for optimizing class limits for landform units, verifying soil loss models, comparing the influence of past management on soil loss, verifying the detailed soil map, as well as serving as base information for future and on-going studies in the watershed.



Archive of Agri-Environmental Programs in Ontario, Green Plan, Research, soil erosion, watershed, soil loss, cesium, spatial trend