Study Areas

In 2003-2004, a study on erosion and sedimentation was carried out in four small catchments in Yanting County and Nanchong City of the Hilly Sichuan Basin and Kaixian County of the Three Gorges region based on analyses of deposits of small reservoirs and ponds by using the

137Cs fingerprinting technique. Geomorphologically, Nanchong belongs to the medium hill region of the Hilly Sichuan Basin while Yanting is in the medium high hill region of the basin. Kaixian belongs to the parallel ridge-valley region with low mountains of the eastern Basin (Fig. 5). Geographical and land use conditions of the four catchments are showed in Table 3.

The Wujia Gully and Jiliu Gully catchments are both near the Yanting Ecology-Agriculture Station of Chengdu Institute of Mountain Hazards and Environment, CAS, in Yanting County, and have drainage areas of 0.22 km² and 0.09 km²,respectively. The two catchments have similar physical geography and land use conditions and elevations vary between 420 m and 560 m above sea level. The catchments are underlain by horizontally bedded mudstones, siltstones and sandstones of the Jurassic Penglaizhen Group. The landform typically comprises steep sandstone cliffs with slopes of 25°-30° separated by gentle mudstone and siltstone terraces of <10°. The gentle terraces and the steep slopes account for 1/3 and 2/3 of the catchment area, respectively. The gentle terraces have been cultivated for centuries,

whereas the steep slopes were originally covered by waste grasses but have gradually been afforested with cypress trees since 1970s.

The Tianmawan Gully catchment in Nanchong has a drainage area of 0.19 km² and elevations vary between 310 m and 420 m above sea level. The catchment is underlain by horizontally bedded mudstones and siltstones of the Jurassic Suining Group. The landform typically comprises dozens of small steep cliffs separated by short gentle terraces. The steep cliffs usually have heights of a few meters and are covered by waste grasses with widely dispersed young cypress trees, while the gentle terraces with slopes of <10° and with lengths of 10-30m are mostly rainfed lands. \

The Chunqiu Gully catchment in Kaixian County has a drainage area of 0.58 km² and elevations between 190 m and 400 m above sea level. The catchment is underlain by mudstones and sandstones of the Jurassic Shaximiao Group with an inclination of about 30°.

The landforms are characterized with inclining bedded rocky low mountains which are typical in the parallel ridge-valley region of the Eastern Sichuan Basin. The catchment and bed rock have the same orientation direction. As the catchment face has same direction with the bedrock dip, bedding slopes occupy most of the catchment. Soils are thin on the bedding slopes and some of the slopes are bare. Part of the bedding slopes are rainfed lands and others are waste grass lands.

In all four study catchments the soils on the slopes are purple soils. These purple soils originated from different strata and have considerable differences in erosion resistance. Due to different proportions of sandstones, the purple soils of the Penglaizhen Group and the Suining Group have relatively high and low erosion resistances while the resistance of the Shaximiao Group is in between. As the size of the studied catchments is less than 1 km², the four small catchments have little valley areas and relatively high longitudinal channel gradients of 10-20%. Reclamation ratio in the Tianmawan Gully is 0.45, while the ratios are 0.25 in other three catchments. Annual precipitations are 1100 mm in Kaixian, 1010 mm in Nanchong and 826 mm in Yanting, respectively, 70% of which occur in the wet season from June to September.

A pond with an earth dam of 4-5m high was constructed at the outlet of each catchment during the period from 1949 to 1956 and had a storage volume of 1.5-5.1×10⁴ m³. The dams were made up of soils dug from the valley bottoms within the ponds. The four ponds have simple water delivery facilities of weirs or bottom culverts with intakes. The storage water in the ponds is used for irrigation in spring and summer. The two ponds in the Chunqiu Gully and in the Jiliu Gully have no flood spill ditches, while the pond in the Wujia Gully catchment has a ditch but it was seldom used. The delivered sediments from the upstream catchments have been predominantly deposited in the ponds in the three gullies. The pond of the Tianmawan Gully has a flood spill ditch, which has been used to prevent flood water with high sediment concentrations from entering into the pond of the Tianmawan Gully for preservation of the limited storage capacity, especially from 1981 onwards when the land use responsibility system was introduced in the rural area of China. However, before 1981 the spill ditch was seldom used and the delivered sediments from the upstream catchment predominantly deposited in the reservoir.

TABLE 3. THE BASIC INFORMATION ABOUT STUDY CATCHMENTS CatchmentDrainage area (km2 ) Relative relief (difference between the lowest and highest altitude) (m a.s.l.)

Longitudinal channel gradient (％)

Strata Strata beddingAnnual precipitation (mm)

Cropland ratioDam height (m) Construction time Storage capacity * (m3 ) Wujia Gully, Yanting0.22140 (420-560)11.8

Penglaizhen Group (mudstones, siltstones and sandstones)

horizontal826 0.254.75195625000 Jiliu Gully, Yanting0.09110 (420-530)19.8

Penglaizhen Group (mudstones, siltstones and sandstones)

horizontal826 0.255.0 195515000 Tianmawan Gully, Nanchong0.19110 (310-420)12.7

Suining Group (mudstones and siltstones)

horizontal10100.454.0 194921000 Chunqiu Gully, Kaixian0.58210 (190-400)9.7

Shaximiao Group (mudstones and sandstones)

30o 11000.254.0 195551000 *Storage capacity of the Chunqiu Gully’s pond is estimated from the counter map (1:10000) and the heights of dam, other reservoirs’ capacities are according to interview to local technicians and farmers.

FIG. 5. A sketch map of the sampling locations.