8 What is water holding capacity? The soil is made up of solids and voids or pore spacePore space is made of a water and an air componentThe soil is limited to how much water it can hold, thus water holding capacityWater holding capacity very dependent on soil texture
10 What happens if too much water is applied? Fills all the pore space - saturationwetlandsDrainage downward - Deep percolationwater unavailable for plantsloss of nutrients
11 Is all the water in the soil available for the plants? NOAvailable water is between field capacity and wilting point. These are measured as pressure required to extract the waterTo reduce plant stress keep water extraction to about 50% of the available water
15 Most of the roots, soil micro-organisms and available plant foods are concentrated in the upper one or two feet of the soilFrom the preceding slide we can see that Seventy percent or more of the water used by grasses and legumes is taken from that area also.
16 What determines how fast the plant uses water? Growth stageLocationClimatewind, temperature, humidity, Solar radiation, ground temperature
17 Consumptive useVarious methods have been determined to predict crop water useMost are not direct measurements but use empirical methodsWhere can this information be obtained?Local NRCS officesState department of Water ResourcesInternet
19 Irrigation Frequency We know how much water the soil holds We know how much and how fast the plant uses the waterWe can estimate how often and how much water to apply
20 What is the Goal? Maximum production If the interval between irrigation is too long the grasses go dormant and production and quality are lostCurrent research shows that alfalfa-grass on deep medium-textured soils decline occurs after 15 days. Clover is approximately 7 days
22 The goal of IrrigationReplace used water with as little waste as possible (runoff and deep percolation)Runoff carries pollutants in to other water bodies, has a bad public perceptionDeep percolation - not visible, but can create temporary water tables, drive out soil air and can leach plant foodsToo much water can also reduce yields as shown in the following graph
24 Irrigation Frequency and Grazing Schedules Optimum irrigation frequencies as well as the amount of water to be applied each irrigation has been determined for most crops and soilsIt must be correlated with the grazing schedules to be used effectively
25 Dry out timeEach irrigation must be scheduled to allow adequate dry-out time before stock are admitted.Dry-out periods of 3 to 4 days are adequate for most soilsWithout dry-out time compaction and damage to the plants
27 The dry-out period plus the number of days each pasture is to be grazed (grazing period) within the rotation cycle plus one day to irrigate should be equal or less than the planned irrigation frequency
30 Once Irrigation cycle equals grazing cycle Don’t adjust irrigation interval to meet seasonal water needs.In cooler months that require less water - just apply the water for shorter time periods instead of adjusting the irrigation interval
31 Studies show...1. Shortening the irrigation interval requires an increase in the number of pastures and a reduction in the grazing interval2. That 2 or more pastures can be watered at a time when there are an even number of pastures or when the grazing period is a multiple of the irrigation interval. Works good for when water is delivered on a rotational basis.
32 3. That a different pasture is watered each time for odd number of pastures unless the grazing period is a multiple of the irrigation interval. Works best with sprinklers or water delivered on a demand basis.4. That flexibility in irrigation intervals and grazing periods increases as the number of pastures are increased.
34 Details Given: 40 acres, deep medium textured soil Grass clover pasture, Water use 0.20in/dSoil water 2 in/ft, root depth 3ft flow rate 100 inches, Soil intake .3in/hrFind: Irrigation frequency, Grazing period,number of pastures, grazing cycle, andirrigation cycle, irrigation set times
35 Solution:Step 1. Find irrigation frequencySoil water x root depth= total water2in/ft x 3ft =6inUse 50% of total water 6in x .5 = 3in3in available waterIrrigation frequency = 15days
36 Step 2. From Tech note select a dry out period of 3 days grazing period = 15days - 1 day - 3days = 11 daysStep 3. From tech note select regrowth periodStep 4. Calculate the grazing cyclegrazing period x number of pastures = 3x11 = 33 daysDoesn’t quite match
37 adjust drying time to 4 days and regrowth time to 20 days. this gives a grazing period of 10 daysand a grazing cycle of 30 daysGood !!
38 P1 P2 P3 14 days 14 days Irrigation Irrigation Irrigation 10 days
39 Step 5. Calculate set times If you are sprinkling that’s fairly good. Use a12 hour set.If you are flood irrigating then you need to add theadvance time for the water to get from the head ofthe field to the end. 2 hours just might do it, so 12hours set would probalby work for both.
40 Now let’s check efficiencies 2 cfs = 4 ac-ft/day soor 83% water each pasture in two sets 12hrs each
41 Summary Irrigation frequency 15 days Dry- out period 4 days Grazing period daysRegrowth period daysGrazing cycle days3 pastures acres2 set per pasture12 hours per set3 inches of water replaced at 83% efficiency