Sloping Land Improvement Project
Rach Rat catchment, Binh Phuoc Province, Vietnam.
Funding
British Council.
Partners
Sub-institute of Geography, Ho Chi Minh City, Vietnam.
Senior Researchers
Professor Ian Douglas (co-ordinator, University of Manchester).
Ms Julia McMorrow (University of Manchester).
Dr Sarah Lindley (University of Manchester).
Mr Nguyen Van De (co-ordinator, Sub-institute of Geography).
Co-Researchers:
Mrs Tran Thi Van.
Miss Dao Kim Nguygen Thuy Binh.
Mr Le Huu Thanh.
Mr Nguyen Tho.
Aims
- To develop a pilot GIS to estimate soil loss under different land management scenarios using land cover maps derived from remotely sensed images, terrain images and ground data on soil erodibility and loss.
- To analyse change in tree cover over time from remotely sensed images.
- To increase the capacity of the Sub-Institute of geography by training young Vietnamese scientists in these methods.
- To apply these methods to manage steep soils, increase agricultural productivity and improve the standard of living of local communities in Binh Phuoc Province.
Study area
Location:
The Rach Rat catchment lies approximately 120 km north of Ho Chi Minh City, in the vicinity of Dông Tãm and Tãm Phuoc settlements in the Dong Phu District of Binh Phuoc Province (newly established from Song Be Province).
Topography:
The study area is an undulating basalt plateau with steep sided valleys incised into the underlying easily eroded, clayey sandstones. Floodplain development is restricted to third or fourth order streams. Steep, normally dry zero order streams cut back into the convex slopes. Thus, the most erodible rocks tend to outcrop on the steep, convex slopes descending towards stream channels.
Climate:
The climate is tropical monsoon with a marked rainy season from April/May to December, followed by the dry season. The highest rainfall is in July and October. Mean annual rainfall is 2044 mm, humidity 78° and temperature 26° C. Flooding is common in the wet season.
Land use:
The natural cover was once tropical monsoon forest. In the years leading up to 1986, the forest cover was depleted by small scale logging and agriculture. Since 1986 clearance for agriculture has been on a massive scale. It marked the start of the transition from a command to a capital economy, known as 'Doi moi', which permitted settlers to claim land by clearing for small-scale agriculture. Burned tree stumps are now all that remain (Figure 1) with strips of bamboo forest along steep uncultivable valley sides.
Perennial tree crops such as cashew, coffee, banana, and longan are grown on the plateau in 4 to 5 ha plots (Figure 2), alongside larger and much longer established rubber plantations. Annual crops are grown during the wet season, with land left fallow in the dry season. Two crops of maize are sown in May and September and harvested in August and December. Maize is also grown below young cashew trees up to canopy closure at four years old. Cassava is planted at the start of the wet season in May and harvested in December. The whole plant is pulled up to harvest the root, so the soil surface is significantly disturbed and the soil erosion potential increased. Wet padi rice, is grown on the floodplain. It is planted in the wet season and harvested when the fields dry out in the dry season. The seasonality of the crop calendar produces marked seasonal changes in reflectance on satellite images (Figure 4 and Figure 5).
Soils:
Four soil groups occur in the study area: (i) Ferralsols on the basalt plateau are relatively fertile; (ii) Ferralsols over schist on valley sides with >15° slope are stoney, thin, low in organic matter and have lateritic horizons; (iii) localised areas of black Andosols, found on tuff and volcanic ash in craters; (iv) Fluvisols, found on the alluvial floodplains.
Soil erosion:
Soil erosion is a major problem (Figure 3), but the newly-formed province has few scientific resources to devote to it. Three needs have been identified which would help to protect soil resources and so alleviate rural poverty. First, evidence of the link between cause (farming practice) and effect (soil erosion) is needed. Second the benefits of simple, local scale changes in land cover and farming practice in reducing soil loss must be demonstrated, including bunds and contour planting. Third, given the land use and terrain, a cost-effective way of estimating soil erosion potential over large areas is required.
Data and methods
Satellite images:
Landsat Thematic Mapper (TM) images were available for the end of the wet season (22 December 1999), (Figure 4) and the end of the dry season (15 April 2001), (Figure 5). A 1989 TM image is available to study long- term change in forest cover.
The images were being geometrically corrected and co-registered for multi-temporal analysis. Training and test data are being collected to produce a land cover classification from the co-registered wet and dry season images. A Global Positioning System (GPS) was used to accurately locate the sample areas and to collect points to use in geometric correction of the satellite images. The importance of using two dates is demonstrated by the significant changes in false colour (and thus reflectance) on the two TM images. Much more exposed soil (blue tones) is seen in the April image where wet season annual crops of maize, cassava and rice have been harvested and are about to be replanted. The perennial tree crops, cashew, coffee, banana, rubber, teak and fruit trees such as longan remain red in both images. Slight changes in reflectance due to seasonal changes in leaf pigments help to discriminate between them. For instance, rubber has a flush of new, brighter green leaves in December. The oldest leaves yellow in March and fall in April, but the tree is not completely deciduous with leaves of different ages growing together.
Terrain data
A digital elevation model (DEM, Figure 6) has been constructed from contour data digitised from a 1:40,000 topographic map. Its accuracy was tested against a DEM derived from a SPOT stereo pair of panchromatic images. The DEM was used to generate spatial data on terrain properties contributing to soil erosion such as slope and slope length.
GIS database
A geographic information system (GIS) data base was being compiled, consisting of: vector layers digitised from topographic maps and field survey (roads/tracks, drainage, contours, farms); raster layers (land cover from remotely sensed images, DEM, slope, etc.); socio-economic data complied from interviews with families.
Soil erosion data
Soil erosion on slopes under different land covers was monitored using an erosion bridge and erosion plots. The erosion bridge is a portable device consisting of a 1.5m long rigid bar, which is mounted on fixed stakes whose height does not change over time. The distance between the ground and the bar gives the micro-topographic profile. Repeated measurements allow the magnitude of loss or gain in soil to be quantified. Sites were chosen to include areal sources such as agricultural plots and linear sources such as unmetalled roads.
Erosion plots were constructed on different land cover types and farming practices. For instance, one plot had cashew with cross-slope soil bunds to trap sediment and a second had cashew with grass undergrowth (Figure 7), so that the effect on soil loss of simple changes in agricultural practice can be demonstrated. Samples for the runoff were collected daily by the farmer at the 'pilot farm', where the plots are located and taken fortnightly for analysis of sediment yield in Ho Chi Minh City. This gave the local community a degree of involvement in, and ownership of, the work. A community workshop was planned in the village to present the results and give the community an opportunity to express their views on the soil erosion issue.
GIS modelling of soil erosion
Tenataive relationships were established between soil erosion, terrain and land cover for the erosion plot and erosion bridge measurement points. It proved difficut to extrapolate these over the study area with the resources available. The original aim was to use land cover images prepared from the remotely sensed images and terrain images derived from the DEM to estimate erosion for the catchment, and from this, the effect of changing land cover and land use practice on soil erosion. More monitoring plots would be required, especially given the diversity of land cover.
Output
Nguyen, V.D., Douglas, I., McMorrow, J., Lindley, S., Dao, K.N.T.B., Tran,. T.V., Le, H.T. and Nguyen, T. (2008) ‘Erosion and nutrient loss on sloping land under intense cultivation in southern Vietnam’. Geographical Research, 46 (1): 4-16.
Dao K.N.T.B, Le T.V.P, Douglas. I , Nguyen V.D, McMorrow J, Lindley. S, Tran T.V, Le H.T, and Nguyen. T (2008) Local knowledge and economic realities affecting soil erosion in the Rach Rat Catchment, Vietnam. Geographical Research, 46 (1): 17-26.