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從全世界河流、湖泊和地下蓄水層汲取的供人類使用的淡水中，將近70%是用於農業，因此節水的最大潛力在於提高農業灌溉效率。減少10%的灌溉用水，我們就能夠使世界各地的家庭用水量增加1倍。我們可以改用節水的灌溉系統，停止耕種最貧瘠的、最陡峭的土地，轉而種植耗水量較低的植物（這可能會要求改變政府對某些農作物的津貼額）；實行適當的農業土地排水和土壤管理辦法，減少化肥和殺蟲劑的使用。
一般而言，政府以很高的津貼優惠價格向大型商業性農民供水，這就削弱了節水的必要性，鼓勵了浪費行為。這種做法已導致農民廣泛使用浪費水資源的灌溉設備。據研究顯示，農業灌溉用水中只35~50%真正用在農作物上。大部分的水通過沒有鋪襯裏的灌溉渠道浸入土壤，或從水管洩漏出來，或在到達農田之前就蒸發掉了。儘管從低效的灌溉系統中損失的部分水，會重新返回可再次抽取的河流或地下蓄水層中，但其水質因殺蟲劑、化肥和鹽份的摻入而定是下降了。這實際上就是商業性農業「用」水的另一種方式：對水造成污染，使之不能再被飲用。在商業性農業盛行的地區，從農田中流出的水已經污染生活供水，使得水中的有毒物質達到危險的程度。
計劃不周或建造不好的灌溉系統不僅會破壞水質，還會由於土壤鹽化而損害土壤的作物生長能力，而且這是不可恢復的。尤其是在乾旱地區，天然的鹽會在被灌溉過的土壤中累積。排放不暢的灌溉水會污染供水，提高地下水位，使地下水位達到作物的根區，浸透土壤，淹死農作物。世界各地約有8千萬公頃農田因土壤鹽化和浸透而退化。
轉而使用節水的灌溉系統可以最大限度的節省農業用水（專家聲稱，水滴灌溉的潛力是可以節省40~60%現在所用的農業用水）。最經常的節水灌溉系統是各種水滴灌溉（也稱作「微灌溉」）。傳統的灑水裝置在農作物上方噴水，不僅得到灌溉的土地超出了農作物生長所需的土地範圍，而且由於水分蒸發而損失了更多的水。然而，水滴灌溉是直接向植物的根系提供少量的水，植物的根部可以吸收這些水。通過滴水裝置給每株植物傳遞水分，或穿洞挖管道，安裝在地表或地下。這將水分蒸發的損失，保持低量，而提高用水效率至95%。
儘管到1991年之前，世界各地已有約160萬公頃農田採用水滴灌溉，但這仍不到全世界灌溉土地面積的1%。一些國家將水滴灌溉列為全國優先推廣事項，例如以色列總灌溉面積的50%都採用水滴灌溉。但是很明顯，這只是個例外，大多數乾旱的國家還有很長的一段路要走。
另一個有發展前途的灌溉系統稱做「低能耗高精度應用法」（LEPA），這種灌溉系統對傳統灑水裝置進行實質性的改進。LEPA方法通過滴管將水運到農作物，這些滴管從灑水裝置的臂狀噴嘴延伸出來。根據Johns Hopkins人口資訊專案（美國）發表的報告『缺水世界的解決方法』，若和適當的節水耕種技術一起使用，這個方法還能將效率提高到95%。由於這個方法在低壓下使用，和傳統的灌溉系統比較起來，能量成本可以降低20~50%。美國德克薩斯州的農民已經利用LEPA改進傳統的噴水裝置，他們報告說產量增加了20%，而且他們的投資成本在一兩年內也就收回來了。
另一項日益盛行的做法是，在附近的農場再次使用城市廢水種植植物和水果（詳情請見原手冊第41頁以後內容）。如今，15個國家至少50萬公頃農田是用處理過的城市廢水灌溉的，這種水通常稱做「褐水」。以色列的褐水專案是所有國家中最大的。以色列絕大多數的污水都經過淨化，重新用來灌溉20,000公頃農田。在厄立特里亞的阿斯馬拉，三分之一的蔬菜都是用處理過的城市廢水灌溉的。在尚比亞的盧薩卡，該市一個最大的非正式居民區的作物，就是用附近沈澱塘裏的污水灌溉的。
版權歸屬 國際河網IRN，環境資訊協會（陶俊 譯，吳國玢，李瑞玉 審校）
中英對照全文：http://e-info.org.tw/issue/water/2001/issue-water-irn01090301.htm
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Since agriculture accounts for nearly 70 percent of the world's fresh water withdrawn from rivers, lakes, and underground aquifers for human use, the greatest potential for conservation lies with increasing irrigation efficiency. By reducing irrigation by 10 percent, we could double the amount available for domestic water worldwide. This can be done by converting to water-conserving irrigation systems; taking the poorest and steepest lands out of production; switching to less-thirsty crops (which may require changes to government subsidies for certain crops); implementing proper agricultural land drainage and soil management practices, and reducing fertilizer and pesticide use.
Typically, governments provide water to large commercial farmers at greatly subsidized rates, decreasing the need for conservation and promoting wasteful practices. This has led to widespread use of wasteful irrigation systems. Studies show that just 35-50 percent of water withdrawn for irrigated agriculture actually reaches the crops. Most soaks into the ground through unlined canals, leaks out of pipes, or evaporates before reaching fields. Although some of the water lost in inefficient irrigation systems returns to streams or aquifers where it can be tapped again, water quality is invariably degraded by pesticides, fertilizers and salts. This is in fact another way that commercial agriculture "uses" water: by polluting it so that it is no longer safe to drink. In areas where commercial agriculture is prevalent, runoff from farms has poisoned water supply with dangerous levels of toxics.
Poorly planned and poorly built irrigation systems not only harm water quality, but can also irreparably harm the crop-growing capability of the land through salinization. Especially in arid areas, salts that occur naturally accumulate in irrigated soils. Poorly drained irrigation water can pollute water supply, and raise the groundwater table until it reaches the root zone, waterlogging and drowning crops. Globally, some 80 million hectares of farmland have been degraded by a combination of salinization and
waterlogging.
Switching to conserving irrigation systems has the biggest potential to save water used for agriculture (experts say drip irrigation could potentially save 40-60 percent of water now used for agriculture). The most common water-conserving irrigation systems are some form of drip irrigation (also called micro-irrigation). Conventional sprinklers spray water over crops, not only irrigating more land than is needed to grow the crop but also losing much to evaporation. Drip irrigation, however, supplies water directly to the crop's root system in small doses, where it can be used by the plant's roots. Water is delivered through emitters that drip water at each plant, or perforated piping, installed on the surface or below ground. This keeps evaporation losses low, at an efficiency rate of 95 percent.
Although by 1991 some 1.6 million hectares were using drip irrigation worldwide, this is still less than one percent of all inigated land worldwide. Some countries have made drip irrigation a serious national priority, such as Israel, which uses drip inigation on 50 percent of its total irrigated area. But clearly it is the exception, and most dry countries have a long way to go.
Another promising irrigation system, called low-energy precision application (LEPA), offers substantial improvements over conventional spray sprinkler systems. The LEPA method delivers water to the crops from drop tubes that extend from the sprinkler's arm. When applied together with appropriate water-saving farming techniques, this method also can achieve efficiencies as high as 95 percent, according to the report Solutions for a Water-Short World, published by the Johns Hopkins Population Information Programme (US). Since this method operates at low pressure, energy costs also drop by 20 to 50 percent compared with conventional systems. Farmers in the US state of Texas who have retrofitted conventional sprinkler systems with LEPA have reported that their yields have increased by as much as 20 percent and that their investment costs have been recouped within one or two years, the report
states.
Another growing practice is to reuse urban wastewater on nearby farms growing vegetables and fruits (more on this starting on p. 41). Today, at least half a million hectares in 15 countries are being irrigated with treated urban wastewater, often referred to as "brown water." Israel has the most ambitious brown-water programme of any country. Most of Israel's sewage is purified and reused to irrigate 20,000 hectares of farm land. One-third of the vegetables grown in Asmara, Eritrea, are irrigated with treated urban wastewater. In Lusaka, Zambia, one of the city's biggest informal settlements irrigates its vegetable crops with sewage water from nearby settling ponds.
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