CIVE 633 - ENVIRONMENTAL HYDROLOGY

SUSTAINABILITY OF IRRIGATED AGRICULTURE

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  • Irrigated agriculture has been practiced for more than 10000 years.

  • Productivity from irrigated lands tends to be higher than rainfed (dryland) agriculture.

  • In the U.S. 25% of all crops originates in 10% of the land.

  • Irrigation in arid or semiarid regions always degrades water quality downstream.

  • Without proper management, the land becomes waterlogged and salinized.

  • Drainage waters from irrigated lands carry salt that requires disposal.

  • Can irrigation be sustained indefinitely?
A LOOK BACK

  • The Egyptians depended on irrigation for 3000 years.

  • In the Indus basin there are drainage problems.

  • Can these problems be solved?

  • Salton Sea developed by dumping agricultural wastewaters in a closed basin.

  • Salinity and sodicity have plagued the development of the San Joaquin valley from the beginning.

  • Discovery of selenium in the biota which fed on Kesterton reservoir has led to renewed evaluation of the disposal of agricultural drainage.

  • Selenium originated in the geology and accumulated in the reservoir.

  • All irrigation enterprises face problems of salinity management.

  • All must consider the safe disposal of wastewater.
SALINITY AND OTHER CONTAMINANTS

  • Salinity problems have been recognized for a long time.

  • Ignoring salinity in irrigation planning is often due to politics rather than to technical understanding.

  • Many engineers have devoted their careers to salinity management.

  • It has long been recognized that salinity management requires removing excess salts from the root zone, i.e., drainage.

  • High concentrations of selenium, molybdenum, arsenic, and boron were found in Kesterton.

  • These minerals originate in the soil or rocks; they are mobilized by irrigation and then displaced with irrigation drainage.

  • Conditions similar to Kesterton are likely to be found elsewhere.

  • Knowledge of local and regional geochemistry becomes paramount to the long-term success of a project.

  • Items of concern include not only salts but also trace elements, excessive quantities of nutrients such as nitrate, and pesticides.

  • Managing irrigation wastewaters has become difficult and controversial.

AN ASSESSMENT

  • Irrigation in semiarid regions must have drainage.

  • A downward flux of water must move through the soil profile to prevent the concentration of solutes in the soil from rising to a level that cannot be tolerated by crops.

  • In some instances, the natural drainage rate is sufficient to meet this need.

  • In other instances, engineered drainage systems are required.

  • Recognition of the need for drainage varies from place to place, depending on the local geohydrology.

  • The concepts of leaching fraction and leaching requirement have been developed to manage salts.

  • Salts must be removed from the system to maintain an acceptable (maximum) concentration of solutes in the root zone.

  • In most natural systems, upland drainage finds its way into rivers and then into oceans.

  • True meaning of rivers: to carry excess salts and transported sediments to the sea.

  • Irrigation tends to accelerate salt displacement.

  • EPA estimated that 1/3 of the salt in the Colorado river can be attributed to irrigation.

  • This is due to new salt (byproduct of the functioning of the biosphere), but also to old salt (fossil salt) that was mobilized by irrigation.

  • Closed basins accumulate salt.

  • Absolute lack of drainage leads to changes; these are biological (more salinity eliminates fish but invertebrate species may survive) and of type of usage (Salton vs. Salt Lake in Utah).

  • All irrigation ultimately degrades water quality offsite for some uses.

  • A permanent irrigation agriculture (benefit) requires the sacrifice of some value elsewhere (loss).
Strategies

  • There are ways to minimize the effects of irrigation on downstream salinity.

  • Limiting the leaching fraction to the amount needed to maintain full growth.

  • Salt comes with water (imported), is already there in the profile (old), and is created by the mere process of crop production (new).

  • All salts need to be removed.

  • Alternative irrigation technologies may minimize but do not eliminate the need for drainage.

  • Desalination is technically possible, but prohibitively expensive.

  • Damage from toxic substances (trace elements) can be more costly than damage from salts.

  • Efficient management of fertilizers and pesticides reduces losses into drainage water.

  • Saline seeps reflect the movement of salt downstream toward the ocean; they reflect geologic changes from closed drainage to open drainage (due to tectonism).

  • Management of saline seeps is technologicaly cumbersome and expensive.

COMPARING THE OPTIONS

  • It may be assumed that the use of land for irrigated agriculture is the preferred choice.

  • Successful irrigation depends on adequate drainage, which implies some offsite loss which tends to counteract the irrigation benefit.

  • Harnessing water for irrigation takes it from other uses (its natural use, which is to transport the salt to the ocean).

  • Irrigation always carries an opportunity cost.

  • Who pays for this cost? Issue is not resolved. Example: Salton Sea.

  • Wetlands in California have been reduced to less than 10% of the original area.

  • Given the objective of maintaining irrigated agriculture, there is a cost (additional drainage) which somebody has to pay for.

  • Bureau of Reclamation was created in 1902 to "reclaim" western lands.

  • Reclaim means irrigate; irrigate means degrade downstream water quality.

  • Benefits of irrigation have been allocated (short term); losses have not been paid (long term).

  • It can no longer be taken for granted that irrigated agriculture should be subsidized and protected.

  • Societal values have changed from 1970 to 2000.

  • There are issues of water rights vs conservation re: Imperial irrigation is being forced to conserve water in view of their usage of water amounts much in excess of their allocated rights.

  • Some of that water will be saved by lining the All-American Canal, resulting in losses to Mexican agriculture.

  • Some of that water will be sold to San Diego.

  • Salton Sea lost, but gained a few millions recently (300?) to "restore."

  • Society no longer considers irrigation to be the preferred use.

  • Should irrigation (and crop production) be transferred overseas, where it is cheaper (and not regulated?)

  • Developed societies such as the U.S. will eventually replace natural services for artificial services.

  • Stability of human-generated systems remains an issue.

  • Political forces drive the system because the issues are matters of policy.

  • Political decisions are influenced by the facts and how they are perceived (which is not the same).

  • Costs and benefits of alternative options should be identified.

  • Holistic, interdisciplinary analysis is a must.

  • In many cases, lack of data implies a risk or uncertainty.

CONCLUSIONS

  • Little question exists that irrigation in semiarid climates can be sustained indefinitely (Yes or No?)

  • Salinization can be avoided by providing adequate drainage.

  • Drainage exacts a price.

  • Drainage water degrades the quality of water along its disposal route.

  • Open drainage, closed drainage, and toxic drainage all exact a prize.

  • Irrigated agriculture must adapt to changing physical and societal conditions to survive.

  • Irrigated agriculture may flourish under the proper circumstances.

  • Reduced subsidies (pay actual cost!) may be the only way to weed out inefficient operations.

  • Problems of irrigation wastewater, salinity, and contamination are of global reach.

 
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