Chapter 13

ObjectivesFacts, issues, & importance of irrigationAmounts of water needed for crop productionApplication methods for irrigationIssues relating to irrigation efficiencyStabilizing water resources

IntroductionWater tends to be determining factor in

profitabilityQuantity & quality of water are critical all

over the worldAren’t we lucky?

Irrigation dates back to the earliest days of farming

Water Resources for IrrigationEarth contains ~327 mi3 of water

97% in oceans2% in glaciers & icebergs.03% in circulated water

Precipitation, transpiration, evaporation Includes all surface water, atmospheric water

vapor, etc. Hydrologic cycle

Supply of water not evenly distributedCan you give some examples?

Water Resources for IrrigationWater Scarcity

Looming problems w/ water supplies Rapidly growing populations Effects of global climate change Conflicts over water resources

Agriculture is, by far, largest user of waterAccount for >70% of water withdrawals

Water Resources for IrrigationFresh Surface Waters

60% of avg. annual river flow in U.S. is in stored reservoirs w/ dams

Dam building in U.S. has basically ended Fewer sites available w/ substantial impact for the

costEnvironmentalists despise dams

Why, what impact do they have? Are they the only ones?

Water Resources for IrrigationWater quality varies

Depends in the geology through which the water flows

Most suitable for irrigation Diversion of surface flow waters cheaper than

pumping subsurface waters Currently ½ of irrigation water from wells

GroundwatersResponsible for greatest increase in quantity of

irrigation water = aquifers May be small to miles long/wide

Water Resources for IrrigationVadose zone – area from soil surface to water

table Includes capillary fringe – depth from bottom of

root zone to top of water table Good for collecting/filtering contaminants

Total groundwater storage ~25x more abundant than surface waters Supplies ~25% of all groundwater used

50% of U.S. citizens obtain their drinking water from ground

95% of rural households depend on it totallyCosts more to use groundwater than surface

water due to expense of pumps & drilling

Water Resources for IrrigationMining Aquifers

Rate of recharge – growing concern Some fill very quickly, some quite slowly

Depends on source of recharge Soils

Water removed > water recharged = mining Low water tables may cause

Dry wells Land subsiding

Water Resources for Irrigation Groundwater supplies ~40% of AZ’s water

Being pumped out 5x rate of recharge AR, OK, TX areas overdraft water ~60%

Irrigation TrendsIrrigated lands comprise ~16% of cropland,

but produce 33% of total harvestChina, India, U.S. – biggest irrigators

70% of grain in China irrigated, 50% in India U.S. – irrigation use declining

Due to costs, water quality, improved efficiency of irrigation systems

Irrigation Water QualityWater quality continuum:

Drinking water – swimming – industry – irrigationIrrigation water can contain considerable

contaminants & still be used, if managed carefully

Turbidity – water cloudiness caused by suspended solids of clays, silts, sands, organic materials Can fill irrigation canals, seal soil pores, clog

irrigation systemsWater temp – limited concern to irrigators

Except if cold enough to reduce growth

Irrigation Water QualityHardness – elements in water – generally

favorable for irrigationBiological Oxygen Demand, Chemical

Oxygen Demand (BOD, COD) – measures of amounts of oxygen & chemicals dissolved in water High BOD’s – decreased oxygen availability

High BOD water can kill fish, decrease soil oxygenPathogenic organisms – disease causing

agents present in waterPesticides – even slightest levels bad in

drinking water

Irrigation Water QualitySalinity

Most important criteria for irrigation waterMost detrimental affect from irrigation water

What problems can it cause? Only takes small amounts to cause detrimental

effectsIf salt levels low, adequate leaching ability in

soil still necessary to rid salts

Irrigation Water QualitySodium Hazard (Sodicity)

High levels of Na causes aggregate dispersionSeals soil pores – decreases permeability

ToxicitiesB – most common toxicity in irrigation water

Relatively low window from deficient to toxic Can easily have trouble either way, depending on

soil

Cl – may cause damage in fruit/vegetable crops

Meeting Water Needs of PlantsConsumptive Use – water evaporated + water

transpired + water in the plant tissueIncreases w/ conditions that favor more

evaporationDailey consumptive use - .1” - .6”/dEvapotranspiration (ET) – evaporated water +

transpired water Easier to measure due to no measurement of plant

tissue water

Meeting Water Needs of PlantsPlant Roots

• Depth influenced by access to water & air• Most water absorbed in upper 1-2’

When to IrrigateRecommended when ~50% of available water lost

from root zoneComputer-aided equipment can help predict when

Can be very expensive, unless you have extensive irrigation

Two mathematical methods also (see pg. 421)Most growers go by gut feel

Methods of Applying WaterBorder-Strip & Basin Irrigation

Ridges direct water through a strip of landWorks well w/ nearly level soils, gentle slope

w/ water flowLoss of 20-45% of water, if no collection &

reuseBasin Irrigation

Each area, tree has own basin Flood basin

Methods of Applying WaterFurrow Irrigation

Oldest form of irrigationWater flow from main (head) ditch through

furrows to end of each rowCrop best if planted on ridgesAccounts for ~40% of all irrigationProblems

Deep percolation Runoff losses Erosion on soils >2% slope

Methods of Applying Water Slope should be <.25%

Many of these lands are laser-leveled

Advantages No moving equipment Can catch excess water More efficient use of water

Methods of Applying WaterSprinkler Methods

Simulates rainfallCan irrigate variety of landCan be portableMore uniform soaking of soilPrecise applicationFertigation possibleCan be expensiveFoliar diseases can be causedWater quality is critical

What can go wrong?

Methods of Applying WaterSprinkler irrigation methods

Lawn sprinklers Solid-set pipe systems Center-pivot Wheel move Travelling gun

VRT can be used to improve efficiencyDrip Irrigation

Most efficient irrigation methodDrip, trickle, microirrigation

Methods of Applying WaterFrequent, slow, small amounts of water appliedLittle/no water lost to surface flow or evaporation

Savings of 20-50% on waterInstallation/maintenance costs highGreatest improvements in yields, w/ least water

usageMore flexible w/ salt levelsCommonly used in orchards, vineyards,

vegetables, greenhousesEmitters can plug easilyDifficult to use w/ field cropping

Special Irrigation TechniquesIrrigating Clay Soils

Difficult due to impermeability, shrink/swell, stickiness

Major problems Inadequate aeration Rapid infiltration through surface cracks Slow infiltration once soil is wet Limited moisture range suitable for tillage

More frequent, smaller quantity irrigations more effective

Irrigating EfficientlyWater Use Efficiency – portion of added

water used by the plantAverage efficiency ~40%

Surface irrigation ~50-65%Sprinkler ~60-70% (85% if well designed &

managed) Low flow w/ drops closer to soil surface 91-96%

How to improve efficiency:Reduce water evaporationImprove equipment, technology, engineeringOptimize total crop management

Increasing Water SuppliesRevert to dryland farming, deep rooted &

drought resistant cropsReduce water pollutionUse catch basins for catching excess water,

plant trees/etc.

Assignment