Maintaining Salt Balance on Irrigated Land
Published Online: 15 JUL 2005
Copyright © 2005 by John Wiley & Sons, Inc.
How to Cite
Kijne, J. W. 2005. Maintaining Salt Balance on Irrigated Land. Water Encyclopedia. 3:677–681.
- Published Online: 15 JUL 2005
The salt balance compares the movement of dissolved salts into and out of an area over a specified period of time. The ratio of the amount of drainage water, percolating from the root zone of an irrigated field, and the amount of water entering the root zone as irrigation and rainfall is the leaching fraction. For steady-state conditions, when the salt concentration of the soil solution remains the same, this ratio equals the ratio of the electrical conductivity (EC) of the applied water to that of the drainage water. The EC of a salt solution is a proxy for its salt concentration and easier to measure than its concentration. These measurements can be made with a conductivity meter and for bulk soil in the field by means of electromagnetic induction sensors.
To sustain irrigated production, the root zone's salt concentration must be controlled by leaching of the salts beyond the reach of roots. Excessive irrigation leads to high water tables and the mobilization of natural salt residues of arid and semiarid lands. Irrigation-induced salt transport contributes to river salinity as illustrated for four rivers.
The salts carried in irrigation water come from salts that are continuously released from rocks and soils by mineral weathering and dissolution. Because of their low rainfall, most arid and semiarid lands are rich in these primary salts. The salts were deposited over time in sedimentary rock layers and are dissolved in the waters of aquifers. Unfavorable salt balances are common in arid lands: The global extent of salt-affected lands is considerable. It has been estimated that worldwide 100 million ha of irrigated land suffer from salinity, 20% of which is severely affected (2). Annually some 2 to 3 million ha of potentially productive agricultural land are taken out of production because of salinization. How much of this land is reclaimed (to various degrees) and then cultivated again is unknown. Wide divergence in the salinity figures is reported by different institutions, probably because of incomplete data and different definitions of salinity-affected lands; e.g., estimates for salt-affected land in India, according to different sources, range from 7 to 26 million ha, or between 17% and 60% of the irrigated land. For Pakistan, the most likely figure is some 40%, Israel 13%, Australia 20%, China 15%, Iraq 50%, and Egypt 35% of the irrigated land (3).
- water table;
- capillary rise;
- arid lands;
- river salinity;
- model studies;
- salt balance;
- deep percolation;
- root zone;