Asia and Pacific UNISDR Informs/Indigenous Knowledge for Disaster Risk Reduction/1. Karez Techonology for Drought Disaster Reduction in China

Submitted by: Weihua Fang, Fei He, Jingning Cai and Peijun Shi

Abstract
Karez is a traditional irrigation water system which is able to make use of underground water efficiently. The Karez system has a long history in the Xinjiang area of China. As a comprehensive system, Karez is composed of four primary components: vertical wells, underground canals, a surface canal and small reservoirs. Because of the Karez system, Turpan, a basin located in the arid area of Northwestern China, is well-known for its wide variety of agricultural products. In the Turpan area of Xinjiang, Karez is still being used to supply water resources for irrigation and domestic uses. At present, modern technology has been integrated into the traditional Karez system to further reinforce the successful traditional practice.

Background
The Turpan depression, with a height of 32.8m, is located in the Turpan basin which is the second lowest basin in the world. It is surrounded by several high mountainous areas (3500~5000m) which are covered with glacier or permanent snow. The minimum height of Aiding Lake, at the south of the basin, is around -155m, which represents the lowest lake in China. Underground water is abundant in the shallow underground water-bearing layer. The terrain of the mountains around Turpan basin is mainly formed by hercynian movement at the end of the Paleozoic period. It is hard and fractured and therefore fissured water is easily formed. The exposed rocks of Flaming Mountains mainly consist of sandy conglomerate and mud stones of Jurassic, cretaceous as well as Tertiary. Therefore, the geological condition of Turpan district is fit to construct an underground canal with little reinforcement for amassing sufficient water resources. The Turpan area is famous for its wide variety of fruits such as grapes, watermelons, and Hami muskmelons.

Story/Event
Turpan is very dry in all seasons and very hot during spring, summer and autumn. The highest temperature recorded is 47.7°C in summer. High temperature and strong solar radiation result in high annual evaporation amounting to 2800~3000 mm.1 Turpan is in an inner land with total annual precipitation of only around 16~17 mm. Because of strong evaporation or the evapotranspiration process, precipitation (rain or snow) falling on the slope of the mountain would evaporate or seep underneath sand and soil before it can converge into a stream and reach the flat agricultural area along the foot of the mountain. Surface water is scarce in most areas. Under such tough circumstances, few plants or animals survive.

Indigenous Knowledge
Karez is a traditional irrigation water system with a long history in Xinjiang area of China which makes use of underground water efficiently. Where farmland is located in the mountainous area, it is built on an alluvial fan or plain. Most existing Karez systems were mainly constructed between the 17th and 20th century. The currently functioning Karez systems are distributed in the dry areas of the southern slope of Mountain Tianshan in eastern Xinjiang, in the Hami district as well as the Turpan, Shanshan and Toksun districts of Turpan Basin. In the Turpan district, 1,016 Karez systems still exist of which 686 are operational. The total length is about 3,000 kilometers. The average depth of the underground canal is 20 meters whilst the utmost depth is 90 meters. The total outflow of the karez systems in Turpan Basin is about 10 cubic meters per second accounting for about 20% of the total diversion water of the basin.2 At present, modern facilities such as electromechanical wells are integrated into the system. The structure of a complete Karez system can be complex but its basic structure is essentially composed of four main components: vertical wells, underground canals, surface canal, and small reservoirs (Figure 1).

Vertical Well
The length of the underground canal varies from around 3 km to 30 km. It is almost impossible to dig such a long underground canal without digging a vertical well, especially in the past agricultural period when no modern equipment was available. Hence, the vertical well is primarily used to assist in digging the underground canals. When the wells are dug, sand and soil are excavated with the help of animals. The major functions of vertical wells are for ventilation, proper orientation of the canal during construction and examination and repair of canals after construction. The distance between vertical wells is generally around 60~100 meters in the upper reaches, 30~60 meters in the middle, and 10~30 meters in the lower reaches. The depth of wells is approximately 40~70 meters, 100 meters in the upper reaches, 30~40 meters in the middle, and 3~15 meters in the lower reaches. Vertical wells are utilized not only to aid in digging underground canals, but also to take out water from these canals after the whole Karez system is completed. An aerial image of vertical wells is presented in Figure 2.

Underground and Surface Canals
Of the two types of canals, the majority of them are underground. The canals under the surface are generally part of a network which enables underground water to accumulate (as illustrated in Figure 3a). Usually the deep soil around the underground canal is very strong and does not collapse easily. However, when the canal is close to the surface, the soil is loose. To prevent the underground canal from collapsing, it is usually reinforced with wooden pillars. At the bottom of the well, both sides are excavated for the underground canal. When the underground canal reaches the farmland, it becomes a surface canal and is linked to a small reservoir or directly connected to a system of water channels for irrigation (Figure 3b). Usually the surface canal is short to limit evaporation.

Small Reservoir
Water resources are collected in small reservoirs which can be adjusted for water level and temperature. Constructing reservoirs increase the water level so that a larger area of farmland is irrigated. Moreover, the water stored in reservoirs receives sunlight which increases their temperature. The warmer water is more appropriate for irrigation, since the low temperature of water due to snow melting or because of its underground source may do severe harm to crops. A variety of simple tools are employed for building Karez systems. These include an excavating hoe, a planning hammer, a basket, a winch and an oil lamp (as displayed in Figure 4). The excavating hoe and planning hammer are used for digging passages underground. The basket and winch are used for carrying soil and sand. Iron oil lamps with an arrow for orientation are used for digging underground canals. The lamp can also be conveniently fixed on canal walls. Currently in Hami city of China, a sunlight reflecting mirror is also used for orientation. In constructing the different components of the Karez system, it is vital to ensure that along the underground canal, water resources are sufficient. A crucial first step is to find water sources in the upper reaches and the depth of water estimated in accordance with the location of farmland. The location for digging the well can then be decided. Subsequently, wells and canals can be built gradually from the lower to the upper reaches following the water source.

Lessons Learned
The Karez system is a proven and effective indigenous drought reduction technology that is still in use. This traditional knowledge has several advantages as follows:

1. Support by Earth’s Gravity. Since Karez takes advantage of topography to divert deep subsurface flow through an underground canal to land surface for gravity irrigation, the cost for water-lifting equipment and its maintenance are almost negligible.

2. Stable Outflow. The major water sources of the Karez system are melting snow and underground water. The underground canal can minimize high evaporation in the windy Turpan district hence, the impact of climate change is small. In addition, problems with sand blast can be avoided in the underground canal. All this makes the Karez system able to provide stable water resources, though total water volume may not be very large. As observed, there has been a very stable population for thousands of years in Karez areas, regardless of environment changes.

3. High Water Quality. Melting water from snow enters the system and the soil provides a very good filter to remove polluted materials. Unlike water channels on land, the underground canal minimizes water pollution and at the same time is rich in minerals. The water quality is suitable for drinking and domestic use.

4. Construction with Simple Tools. Most Karez systems are built with simple tools and do not require complex equipment.

Conversely, the Karez system has constraints. There is a spatial limitation on its construction and use. It is only applicable in areas with stable underground water supply and hard soil. Some Karez systems were built around Guanzhong Plain in Central China in the Han Dynasty which did not last because the underground canals collapsed. Furthermore, the volume of water in the Karez system may change with the season although there is little variation in the volume daily. In the summer, there are sufficient water resources if the water source is from melting snow. In the spring, Karez water is limited, while in autumn and winter it is substantial. This is sometimes in discord with water demand for agriculture.

There is thus a need to strengthen the traditional Karez system with modern technology. Its value as an efficient indigenous drought reduction technology should not be ignored. Instead, this traditional knowledge should be improved and reinforced with modern technology. Its widespread use promoted in the face of severe drought disaster in the future.