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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?
- 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.
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SALINITY AND OTHER CONTAMINANTS
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- 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.
- 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.
- 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.
- 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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