Discussion

Streamflow of the Tuoshigan-Kumalake River decreases along the main stem with a 9,200×10⁶ m³/year discharge at the upstream boundary and 7,700×10⁶ m³/year discharge at the downstream boundary. The dominating hydrological processes are related with the river water dispersion to the alluvial plain, i.e. dispersed flow, rather than runoff concentration to the riverway. Intensive surface water-groundwater interaction and human disturbances make these processes complex and limits our knowledge of the hydrological cycle in the alluvial plain. Four years of fieldwork was performed by our project group to investigate the agricultural ecosystem inside the Tuoshigan-Kumalake River plain.

Mean annual evaporation over irrigation area in two sub-areas, 276×10⁶ m³ and 318×10⁶ m³ respectively, corresponded to only 46% and 43% of the ditch diversion respectively, showing that the local irrigation system, using long earth ditches on sandy soil and a relatively small flow of water, is inefficient. However, the ditch losses contribute to recharging groundwater (787×10⁶ m³/year) and raise the water table. The artesian springs, which originate from the groundwater in non-irrigation area, partly recharged by ditch and irrigation area, are usually led back into ditch system, making the water transformation inside the plain more complicated.

Mean groundwater flows from the irrigation area to the non-irrigation area in the two sub-areas, 298×10⁶ m³ and 270×10⁶ m³ respectively, stood for 63% and 40% of the non-irrigation area water supply respectively, indicating that the }surplus} irrigation and the losses from the irrigation system were transferred to natural ecosystem through groundwater. Tight hydrological connections between agricultural and natural ecosystem suggest that any changes in the irrigation system, such as the building of large civil works, might affect the surrounding natural ecosystem and human water supply from springs negatively.