Diferencias

The climate in the area is semi-arid and continental, with occasional heavy thunderstorms in summer. At Ansai town, 5 km from the Danangou catchment, total average yearly rainfall was 513 mm over the period 1971-1998 (data from Ansai County Meteorological Station). Most of the rain (72%) fell in the period June-September, and all heavy storms occur in that period. Only during these large storms will runoff occur in the catchment. On average, three to four storms each year are large enough to cause runoff, but the actual number varies widely from year to year. Figure 3.10 shows the average monthly rainfall amount at Ansai. It shows the concentration of rain between June and September and also shows high standard deviations, which indicates large inter-annual variability. The large inter-annual variability is partly caused by the influence of the front of the summer monsoon, which does not advance equally far inland in all years. Figure 3.11 shows the daily rainfall amounts at Yan’an for different return periods. The climate at Yan’an is comparable to that at Danangou, which is only 40 km away. Elevation and topographical location are also similar. Data were provided by Beijing Normal University and cover the period 1971-1995. The partial duration series technique (Ven Te Chow et al., 1988, p 383) was used to get information on return intervals for all days with rain of more than 13

Figure 3.10 Monthly rainfall at Ansai over the period 1971-1998. Data from Ansai County Meteorological Station

Figure 3.11 Daily rainfall amount as a function of return period, Yan’an. Based on data provided by Beijing Normal University. A partial series technique has been used on all daily rainfall

amounts over 13 mm

mm, instead of only on the maximum daily rain of each year. Figure 3.11 shows different types of distributions that have been fitted to the data. From the chart it is apparent that the lognormal distribution gives the best results. It gives a good fit to the data, except for the largest daily rainfall amount. This is normal; since the length of the data series does not allow for calculated return intervals of more than 25 years, while in reality an event

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0

jan feb mar apr may jun jul aug sep oct nov dec month precipitation (mm) average standard deviation y = 16.204Ln(x) + 50.796 R2 _{= 0.9701} 0 20 40 60 80 100 120 140 160 0.1 1 10 100

return period (years)

daily rain (mm)

data

extreme value type I log-pearson type III log (data)

with larger return interval might well occur in such a period. From the chart the return period of a daily rainfall of certain magnitude can be read. For example, a daily rainfall amount of 50 mm will occur once every year. No data on storm rainfall are available, so that daily rainfall amounts have to be used. This limits the usefulness of the chart

considerably, since storms that have equal amounts of rain might have very different intensities and thus very different return periods.

Winters are dry (figure 3.10) and cold because of the influence of the Siberian high- pressure system that causes the winter monsoon (Huang et al., 2000). Data from the Ansai County Meteorological Station show that over the period 1971-1998 the average temperature in January was –6.8 degrees centigrade. Over the same period the average temperature in the warmest month (July) was 22.4 degrees, and yearly average

temperature was 8.9 degrees.

The loess soils of the Danangou catchment are in principle permeable. Infiltration rates will however be significantly reduced by sealing and crusting of the soil surface.

The loess soils also have high porosity and have a good water holding capacity (Messing et al., in press a). This means that if water infiltrates it is likely that most remains in the upper part of the soil, so that it can later be used by plants. Deep drainage of water into the loess probably only occurs along fissures in the loess (these are common) and by sporadically occurring soil pipes. The existence of a small groundwater reservoir is indicated by a very small, but continuous, leakage of water along the bedrock planes and along the bedrock-loess boundary. In some places there is dripping water, while in others there are wet zones that show deposition of salt crystals from the evaporating water. The farmers have built several cisterns that collect the dripping or seeping water. During very wet years water pressure might build up sufficiently to induce large mass movements, probably aided by progressive weathering of material along the bedrock-loess boundary. In most places the water table is at great depth (several tens of metres at least).

During rainfall events only overland flow (and pipe flow) will reach the outlet. Infiltrated water will probably not reach the outlet at all, as indicated above, and given the fact that potential yearly evapotranspiration is several times higher than yearly precipitation. 2.6 Land use

Agricultural land is cultivated in a labour intensive way. The reasons for this are both economical and environmental. The farmers generally lack funds and cannot afford any mechanical equipment. Fertiliser is applied on a limited scale, but essentially all farm work has to be done by human or animal power. Besides, the steep slopes make

mechanisation almost impossible. For ploughing donkeys or oxen are used, weeding and harvesting are done manually. Figure 3.12 shows ploughing with an ox on a steep slope, while figure 3.13 shows grinding of corn by a mule.

Croplands are mainly located on the flatter areas along the hilltops and also on the flatter areas lower down. Some families also have alluvial land along the Yan river. The

Danangou stream drains into the Yan river. The alluvial land is not located inside the Danangou catchment itself. The alluvial land is the best land, because it is level and can be irrigated with water from the Yan river. In the catchment itself the farmers prefer the plots closest to their home, so that croplands on top of the ridges are more likely to be left fallow. Still, plots on slopes of up to about 60% are used as arable land. Plots are of small size. The main crops in the catchment are pearl millet, foxtail millet, soy bean, potato, buckwheat and maize. Several other crops are also present: black beans, green beans, hemp, sorghum, and sunflower. At present about 40% of the catchment is used for

growing crops. This area is likely to decrease in the future as the Chinese government has formulated new policies about the Loess Plateau that aim at reducing the maximum permissible slope angle for cropland to 15 degrees.

Figure 3.12 Ploughing with an ox on a steep slope

About 40% of the catchment is wasteland. Until recently it was used for grazing goats, but this practice has virtually ceased after it was prohibited by the Chinese government as part of a new Loess Plateau policy that aims at erosion reduction. The wastelands are generally rather poorly vegetated grasslands with small shrubs and they are located on the steepest slopes in the catchment. These are the slopes of the main gullies, which can be as steep as 70 degrees.

The remaining 20% of the catchment is occupied by woodland, orchard, fallow land, vegetable gardens and houses. Some of the upper gullied valleys have been revegetated with woodland in an attempt to limit erosion. The most common trees are willows and locusts. Orchards are located at low elevation. Apple is most common, but pear, apricot, peach and Chinese date are also present. The new Loess Plateau policy aims at increasing woodland and orchard areas. Vegetables are usually irrigated either by hand (buckets) or using small channels that start at the springs in the catchment.

Figure 3.13 Farmers grinding maize with a millstone. A mule is used to rotate the upper millstone. The mule is blindfolded to prevent it from becoming dizzy

Figure 3.14 shows a land use map made in 1999, while table 3.3 shows the areas

occupied by the different land use types from 1998 to 2000. In 1998 the land use mapping was conducted by RCEES (Research Centre for Eco-Environmental Sciences, Beijing, China), in 1999 and 2000 by UU (Utrecht University, The Netherlands). Mapping by different persons explains some of the differences between 1998 and 1999 & 2000, e.g. concerning orchard and forests. Table 3.3 shows a clear decrease in cropland area accompanied by a similar increase in fallow area. This change is probably caused by a combination of weather conditions and government policy. During dry years less land is cultivated than during wet years, while the crop types are also different. In 1998 the different crop types were not distinguished during mapping, but the 1999 and 2000

mapping revealed that the 10% decrease in cropland area between those years was largely due to a decrease of soy bean (6%), foxtail millet (1.6%) and maize (1.2%). Farmers confirmed that they did not sow soy bean in 2000 because they expected a dry year. Other

Fi

gure 3.14 Land use ma

p of the Danan

crops, like potato, buckwheat and pearl millet were not reduced in area. Buckwheat and pearl millet are sowed late (June), at that time some rain had fallen in 2000. Another cause can be the new government policy, since this policy was presented to the farmers in late 1999 or early 2000. As shown above, this policy is intended to result in a lasting decrease in cropland area, accompanied by increases in forest and orchard areas.

Table 3.3 Land use (%) in the Danangou catchment, 1998-2000

Land use 1998 1999 2000

Cropland 35.8 25.5 15.8

Cropland with small fruit trees 1.1 1.2 1.5

Vegetables 0.1 0.5 0.7

Fallow 7.3 19.9 27.7

Orchard 2.4 1.4 1.4

Shrubland 1.2 1.0 1.1

Forest 11.7 7.9 8.1

Young locust trees 0.0 0.0 0.4

Wasteland 40.5 41.9 42.7

Village * 0.6 0.6

Total 100 100 100

*Not mapped separately in 1998