CMG GardenNotes #263 Understanding Irrigation Management Factors

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1 CMG GardenNtes #263 Understanding Irrigatin Management Factrs Outline: Lcatin f sil misture, page 1 Type f sil, page 2 Water-hlding capacity, page 3 Evaptranspiratin, page 4 Rting depth, page 5 Irrigatin: Hw much? Hw ften?, page 5 Fine-tuning fr the site, page 6 Other factrs influencing irrigatin management, page 7 Tls t evaluate sil misture, page 9 Pr watering practices lead t many cmmn landscape prblems, including irn chlrsis, lw plant vigr, fliar diseases, rt rts, and water pllutin. On a cmmunity-wide basis, landscape irrigatin typically uses twice the amunt f water that the plants actually need. Several cmplex factrs wrk tgether in irrigatin management, including the fllwing: The sil s water-hlding capacity (the quantity f water held by the sil) Evaptranspiratin, ET, (a measurement f actual water use by the plant and lst frm the sil by evapratin). ET is a factr f weather (temperature, wind, humidity, and slar radiatin) and plant grwth. Rting depth. The plant s ability t extract water frm the sil. The plant s water need. Lcatin f Sil Misture Fllwing dry winters r summer drughts, sils may be dry in the tp layers with misture nly in deeper layers. Fllwing extended drught, it is pssible that sils may be dry in deep layers and wet nly in the tp few inches fllwing a light rain r irrigatin. Dry sils tend t resist wetting. Alternating irrigatin applicatins with shutffs t allw water t sak in (cycle and sak irrigatin) may be necessary t wet a dry sil prfile. Irrigatin management is basically applying the crrect amunt f water at the crrect frequency t supply water needs f the plants. Additinal water wuld be wasted as it wuld leach belw the rting zne

2 Type f Sil Sil texture, structure and rganic matter cntent determine the water-hlding capacity and water mvement f a sil. Water cats the sil particles and rganic matter, and is held in small pre space by chesin (chemical frces by which water mlecules stick tgether). Air fills the large pre space. In large pre space, water readily mves dwnward by gravitatinal pull. In small spre space, water mves slwly in all directins by capillary actin. Figure 1 illustrated water mvement in a sandy sil with large pre space and clayey sil with small pre space. [Figure 1] Figure 1. Cmparative mvement f water in sandy and clayey sils Sandy Sil Large pre space dminate sandy sil, giving it rapid drainage. Thus, surface runff f irrigatin water is generally nt a cncern with sandy sil. Water mvement is primarily in a dwnward directin by gravitatinal pull in the large pre space with limited sideward and upward mvement by capillary actin in the small pre space. Thus, in drip irrigatin the emitters must be placed clser tgether than in clayey sils. Sandy sils have a lw water-hlding capacity due t the lack f small pre space. Organic matter, which hlds ten times mre water than sand, significantly imprves the water-hlding capacity f sandy sils. As a pint f clarificatin, plants n sandy sils d nt use mre water than plants n clayey sils. With the limited water hlding capacity, sandy sils simply need lighter and mre frequent irrigatins than clayey sils. Water readily mves belw the rting zne when t much is applied at a time. Clayey Sil Small pre space dminates clayey sil, giving it high waterhlding capacity. Hwever, the lack f large pre space greatly limits water mvement. Water is slw t infiltrate int clayey sil, ften leading t surface runff prblems. Cycle and sak irrigatin is apprpriate n clayey sils t slw applicatin rates and reduce surface runff. In clayey sils, sil structure (creating secndary large pre space) als directly influences water mvement and sil xygen levels. Cmpactin (a reductin in 263-2

3 pre space) further limits water mvement and reduces sil xygen levels, resulting in a shallw rting depth. The ttal water supply available t plants is reduced by the shallwer rting. With higher water-hlding capacity but limited drainage, clayey sils need heavier, but less frequent irrigatins than sandy sils. Watering t ften can aggravate lw sil xygen levels. Because water mves slwly in all directins by capillary actin, drip emitters may be placed further apart than in sandy sils. Fr additinal discussin n texture, structure and pre space, refer t CMG GardenNtes #213, Managing Sil Tilth. Water-Hlding Capacity The terms, saturatin, field capacity, wilting pint, and available water describe the amunt f water held in a sil. [Figure 2] Figure 2. At saturatin water fills the pre spaces. At field capacity air ccupies the large pre spaces while water fills the small pre spaces. At the wilting pint, plants cannt extract additinal water frm the sil. Saturatin refers t the situatin when water fills bth the large and small pre spaces. With water replacing air in the large pre spaces, rt functins temprarily stp (since rts require xygen fr water and nutrient uptake). Prlnged perids withut rt xygen will cause mst plants t wilt (due t a lack f water uptake), t shw general symptms f stress, t decline (due t a lack f rt functin) and t die (due t rt dieback). During summer flding f the Mississippi River in Iwa and Illinis it was bserved that healthy trees were smewhat tlerant f a shrt-term flding perid, whereas trees under stress r in a state f decline were very intlerant. Field capacity refers t the situatin when excess water has drained ut by gravitatinal pull. Air ccupies the large pre space. Water cats the sil particles and rganic matter and fills the small pre space. A handful f sil at r abve field capacity will glisten in the sunlight. In clayey and/r cmpacted sils, the lack f large pre space slws r prhibits water mvement dwn thrugh the sil prfile, keeping sils abve field capacity fr a lnger perid f time and limiting plant grwth

4 Permanent wilting pint refers t the situatin when a plant wilts beynd recvery due t a lack f water in the sil. At this pint the sil feels dry t the tuch. Hwever, it still hlds abut half f its water; the plant just des nt have the ability t extract it. Plants vary in their ability t extract water frm the sil. Available water is the amunt f the water held in a sil between field capacity and the permanent wilting pint. This represents the quantity f water available r usable by the plant. Nte frm the illustratin belw that the amunt f available water is lw in a sandy sil. Lamy sils have the largest amunt f available water. In clayey sils, the amunt f available water decreases slightly as capillary actin hlds the water s tightly that plants cannt extract it. [Figure 3] Figure 3. Relatinship between sil texture and available water Evaptranspiratin, ET Evaptranspiratin, ET, is the rate at which a crp uses water fr transpiratin and grwth plus evapratin frm the sil surface. Primary influences n ET include weather factrs (temperature, wind, humidity, and slar radiatin) and the stage f plant grwth. On ht, dry, windy days, ET will be higher. On cl, humid days, ET will be lwer. In the summer, ET changes significantly frm day t day. T illustrate seasnal variatins, the typical irrigatin requirement fr cl seasn turf in Clrad is given in Table 1. [Table 1] Table 1. Weekly Water Requirement fr Cl Seasn Lawns in Clrad Inches f water (irrigatin and rain) per week Late Early April May June July August September Octber

5 Rting Depth Irrigatin management shuld be taken int accunt the rting depth, adding water t the actual rt area. Rt systems may be cntained r spreading. Annual plants tend t have cntained rt systems, whereas wdy trees and shrubs have mre wide-reaching rts. A newly planted annual flwer r shallw-rted plant cannt btain water frm deeper sil depths. Deep watering f these plants is wasteful. Rts nly grw where there are adequate levels f sil xygen. In clayey r cmpacted sils, where a lack f large pre space restricts xygen levels, rts will be shallw. Plants with a shallw rting depth simply have a smaller prfile f sil water t use. [Figure 4] A plant with deeper rts will need less frequent but heavier irrigatin than the same plant with shallw rts. This, hwever, shuld nt be interpreted as necessarily using less water. Fr example, turf-type fall fescue may rt mre deeply than Kentucky bluegrass (if sil xygen levels allw). With deeper rting, it requires less frequent irrigatins, but irrigatins must be heavier t recharge the rting zne. Actual water-use rates f Kentucky bluegrass and tall fescue are similar. Figure 4. Plants with a deeper rting systems reach a larger supply f water and can g lnger between irrigatins. With deeper rting, irrigatins will be less frequent but heavier t recharge the larger rting zne. In cmpacted r clayey sils, lw levels f sil xygen limit rting depth, thus reducing the supply f available water. Irrigatin: Hw Much? Hw Often? Table 2 illustrates the relatinship f the sil water-hlding capacity, ET and rting depth. These textbk figures make a gd starting pint fr understanding irrigatin management. Mst autmatic sprinkler systems are set t keep the lawn green in the summer. (i.e., set fr the higher summer water need). Withut seasnal adjustments n the irrigatin cntrller the lawn will be ver-irrigated in the spring and fall by abut 40%. This springtime ver-irrigatin is a primary cntributing factr t irn chlrsis

6 Table 2. Irrigatin Summary f a Textbk Sil Sil Type Sandy Lamy Sandy Lam & Clayey Available water per ft f sil inch rting depths Inches f available water and Inches f water t apply per irrigatin (Additinal amunts wuld leach belw the rting zne.) Typical days between lawn irrigatin Spring/Fall (at 1.0 inches/week) 1.8 days 2.7 days 3.6 days Summer (at 1.5 inches/week) 1.2 days 1.8 days 2.4 days 12-inch rting depth Inches f available water, and Inches f water t apply per irrigatin (Additinal amunts wuld leach belw the rting zne.) Typical days between lawn irrigatin Spring/Fall (at 1.0 inches/week) 3.6 days 5.3 days 7.1 days Summer (at 1.5 inches/week) 2.4 days 3.6 days 4.8 days 24-inch rting depth Inches available water and Inches f water t apply per irrigatin (Additinal amunts wuld leach belw the rting zne.) Typical days between lawn irrigatin Spring/Fall (at 1.0 inches/week) 7.1 days 10.7 days 14.2 days Summer (at 1. 5 inches/week) 4.8 days 7.1days 9.5days Fine-Tuning fr the Site The textbk figures are a gd starting pint t understand irrigatin management. When cupled with careful bservatins, a gardener can quickly fine-tune his/her irrigatin schedule t the site-specific irrigatin demands. On a typical July day, if the lawn is using an average f 0.20 inch per day, yu can estimate the water-hlding capacity and rting depth by bserving irrigatin needs. Fr example: If the lawn will g five days n ne-inch f water, and additinal water wn t extend the interval between required irrigatins, the water-hlding capacity (fr this sil and rting depth) is 1 inch. One inch wuld be the maximum amunt f water t apply per irrigatin, as additinal amunts wuld leach belw the rting zne. The ideal irrigatin wuld be 1 inch f water every 5 days

7 If the lawn will g fur days n 0.80 inch f water, and additinal water wn t extend the interval between required irrigatins, the water-hlding capacity (fr this sil and rting depth) is 0.80 inch. This wuld be the maximum amunt f water t apply per irrigatin, as additinal amunts wuld leach belw the rting zne. The ideal irrigatin wuld be 0.8 inches f water every 4 days. If the lawn will g tw days n 0.40 inch f water, and additinal water wn t extend the interval between required irrigatins, the water-hlding capacity (fr this sil and rting depth) is 0.40 inch. This wuld be the maximum amunt f water t apply per irrigatin, as additinal amunts wuld leach belw the rting zne. Irrigatin ptins include the fllwing: The ideal irrigatin wuld be 0.4 inches f water every 2 days. These textbks figures dn t take int accunt expsure, wind r irrigatin system efficiency. They make a gd start pint, but will need adjustments t fine-tune it t the specific site. Fr example: In full shade (nt under large trees), water use culd be 30% lwer. In unusually ht weather r in pen, windy sites, water use culd be 20% t ver 50% higher. In the rting area f large trees, water use culd be 30% t 50% higher (as the tree is pulling water as well as the plants in the shade under the tree). Fr examples, in the authr s landscape, the frnt lawn (pen site with cnstant summer wind) uses 20% mre water than the nrmal ET. While the back lawn (sheltered frm the wind by the huse and wd fence) uses the nrmal ET. S the trick fr efficient irrigatin is t start with the textbk numbers then fine tune them based n bservatin. Based n actual bservatins fr each zne, adjust the run time up/dwn in 10% increment t fine-tune the irrigatin. These examples are based n typical July weather. Fr cler spring and fall seasns, the amunt f water t apply generally remains the same, with a lnger interval between irrigatins. Other Factrs Influencing Irrigatin Management Other factrs als have a direct influence n the actual water-hlding capacity and irrigatin demands, fr example: Expsure The plant s expsures greatly influences water demand. Sun, heat and wind increase water demand. Shade decreases water demand. Water use fr a lawn n a windy, suthwest-facing slpe culd be duble the water use f a lawn in full sun but sheltered frm wind and extreme heat. Sil rganic matter cntent Since rganic matter hlds ver ten times mre water than sand, a sandy sil with gd rganic cntent (arund 4 t 5%) will hld mre water than indicated in the table abve. Over time, clayey sils 263-7

8 with gd rganic cntent may have an imprved sil structure, supprting a deeper rting depth. Previus irrigatin pattern Plants adjust rting depth (t the extent that sil xygen levels allw) t where sil water is available. Frequent irrigatin eliminates the need fr plants t develp a deep rting system. A shallw rting system makes the plant less resilient t ht, dry weather. Stage f grwth The stage f grwth als influences ET. Water needs increase as a plant grws in size during the seasn and peaks during flwering and fruit develpment. Cmpared t the rting system f a mature plant, newly planted r seeded crps dn t have the rt systems t explre a large vlume f sil fr water. Recently planted and seeded crps will require frequent, light irrigatins. In ur dry climate, even xeric plants generally need regular irrigatin t establish. Cnfusin abut plant water requirements can arise frm changing needs as plants mve thrugh their life cycles. Fr example, newly planted trees are extremely intlerant f water stress. Established trees in gd health are rather tlerant f shrt-term water stress. Older trees in decline are intlerant f water stress. General statements abut the ability f trees t tlerate dry situatins need t take int accunt life-cycle stages. Water demand f a plant Plants vary greatly in the demand fr water t 1) supprt grwth, and 2) survive dry spells. (Nte that the tw are nt necessarily related.) Ability t extract water Plants vary in their ability t extract water frm the sil. Fr mst plants, the available water is abut 50% f the sil s ttal water supply befre reaching the permanent wilting pint. Onins are an example f a crp that can nly extract abut 40%. Drught mechanism A similar, but unrelated, issue is the plant s ability t survive n dry sil. Plants have evlved with a variety f drught mechanisms, fr example: Small leaves, waxy leaves, hairy leaves, and light-clred leaves are characteristics f many plants with lwer water requirements. Sme plants, like cacti, have internal water strage supplies and waxy catings. Many plants, like impatiens, readily wilt as an internal water cnservatin measure. Trees clse the stmata in the leaves, shutting dwn phtsynthesis, during water stress. Sme plants, like Kentucky bluegrass, can g drmant under water stress. Kentucky bluegrass slws grwth as sils begin t dry dwn. (Des yur irrigatin management capitalize n this dry-dwn, als reducing yur mwing?) Tall fescue is an example f plants that survive shrt-term dry sil cnditins by rting mre deeply (if sil cnditins allw) t reach a larger water supply. But tall fescue can t g drmant

9 Tls t Evaluate Sil Misture Gardeners have a number f tls available t evaluate the amunt f misture in their sil. Plant bservatin is a gd guide t sil misture. Lk fr clr change and wilting. Fr example, Kentucky bluegrass will change frm a blue-green t grayblue with water stress. Ftprints in the lawn that d nt rebund within 60 minutes are anther symptm t watch fr. Use f an indicatr plant in a perennial flwer bed is als useful. Certain perennials such as Ligularia stencephala The Rcket and Eupatrium rugsum (White Snakert) ften wilt befre ther perennial flwers, indicating irrigatin will shrtly be required. The hand feel methd used when digging in sil is mre evidence f misture cntent. Is the sil pwder dry, medium mist r even muddy? The ease with which a prbe can be inserted can be telling. A screwdriver will punch int the sil mre easily when wet than when dry. Hwever, this can be very misleading, as a clayey sil may be difficult when wet and impssible when dry. A sandy sil may be easy when dry and easier when wet. Sil misture meters are available. A simple, huseplant water meter can be used utdrs. Althugh the exact number reading may give little infrmatin, the verall indicatin f wet r dry is useful. It will read n the wet side when the sil has high nutrients r salts, and n the dry side when the sil is lw in nutrients and salt. Permanently buried sil misture sensrs are available t autmatically activate irrigatin systems when the sil has dried. CMG GardenNtes n Irrigatin Management #260 Irrigatin Management: References and Review Questins #261 Clrad s Water Situatin #262 Water Mvement Thrugh the Landscape #263 Understanding Irrigatin Management Factrs #264 Irrigatin Equipment #265 Methds t Schedule Hme Lawn Irrigatin #266 Cnverting Inches t Minutes #267 Watering Efficiently #268 Hme Lawn Irrigatin Check-Up Authrs: David Whiting (CSU Extensin, retired), with Carl Wilsn (CSU Extensin, retired). Artwrk by David Whiting; used by permissin. Clrad Master Gardener GardenNtes are available nline at Clrad State University, U.S. Department f Agriculture and Clrad cunties cperating. Extensin prgrams are available t all withut discriminatin. Cpyright Clrad Master Gardener Prgram, Clrad State University Extensin. All Rights Reserved. CMG GardenNtes may be reprduced, withut change r additins, fr nnprfit educatinal use. Revised Octber