Water

Annual precipitation in the Dunhuang area is 16-24 mm. Water for irrigation and daily living at the Mogao grottoes is obtained from the Daquan River, which flows through the site. The ultimate source is mostly meltwater from snow in the distant Qilian mountains. The river contains a high concentration of salts. It has long been used to periodically irrigate the trees growing in front of the grottoes, without harmful effects to them.

Drip Irrigation

Drip irrigation systems are more effective and less wasteful of water than traditional surface furrows. A network of closed conduits supplies small amounts of filtered water to the plants according to a regular schedule. Weed growth and water consumption by weeds are limited, because only a small surface area of the soil is irrigated.

The major disadvantage of drip irrigation systems is clogging of emitters and drip lines. Clogging can be physical, chemical, or biological. Physical clogging is caused by particles of sand, silt, clay, and waterborne debris too large to pass through the small openings of the emitters; particles may also be deposited in the lines, reducing water flow. Chemical clogging is caused by soluble salts that precipitate on emitters as water evaporates from the emitter surface between irrigation cycles. Biological clogging is caused by microorganism growth inside the system. Certain species of bacteria and algae flourish inside drip systems and produce deposits, often of iron oxides, that clog pipes and emitters.

To prevent clogging of the system, some preventive approaches were adopted. An in-line filter was installed to prevent clogging caused by particles; a chemical injector was added to the system for application of acid and/or chlorine at the end of each watering period to deal with chemical and biological clogging. The injector can also be used to fertilize the plants. Chlorination is an effective measure against biological clogging. Sodium hypochlorite solution is the easiest form of chlorine to handle and is most often used in drip irrigation systems. However, excessive amounts of chlorine result in injury to young trees and other plants. Tyson and Harrison (1987) have recommended 5 ppm as an effective concentration. However, sodium hypochlorite increases the pH of water, and precipitation of calcium and magnesium carbonate tends to occur. Bucks, Nakayama, and Gilbert (1979) pointed out that when the pH of irrigation water is above 7.5 and high calcium or magnesium levels are present, carbonates precipitate out either in filters, tubing, or emitters. Therefore, it was decided to use acid, if necessary, to prevent the formation of deposits. Sulfuric and hydrochloric acids are the most widely used.