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