Water Management in Pecan Orchards Dr. Jim Walworth Dept. of Soil, Water & Environmental Sci. University of Arizona
Orchard Water Use = Evapotranspiration Soil Evaporation + Plant Transpiration Combined water loss from soil evaporation & plant transpiration
Reference Evapotranspiration (ET o ) Weather Data Physically - Based Model Evapotranspiration from a uniform surface of dense, actively growing vegetation, not water-limited: Represents water demand from the atmosphere
Field-Derived Multiplier K c = Crop Coefficient Ability of Vegetation to Meet/Supply Evaporative Demand (ET o ) Presented as curves that show how crops/plants can supply atmospheric demand (ET ref ) over the course of the growing season or crop cycle.
3D Sonic Anemometer Instruments oriented into main wind Krypton Hygrometer
Scheduling Irrigation Based on Pecan Water Use ET o x K c = Crop ET H 2 O X =? Reference ET o from Weather Station Crop Coefficient (K c = ET c /ET o ) Pecan ET c From Eddy Covariance Tower
Depth of Water (cm) 250 200 175 183 150 123 125 100 50 38 15 Precip ET o ET c (actual water use)
Scheduling Irrigation Based on Soil Moisture Tensiometer Measures soil water matric potential (tension or pull) Range 0 to -80 kpa Advantages: Inexpensive ($US 50-100) Simple Disadvantage: Maintenance, suction breakage
Electrical Resistance Block Measures electrical resistance of soil solution Range: -10 to -200 kpa Advantages: Simple to use Does not break suction (work in very dry soils) Can be left in field Relatively inexpensive (~$US400 for cheapest meter + a few sensors Disadvantages: Irrigation threshold affected by soil texture Not accurate around saturation Electrical Resistance
Dielectric Methods Time Domain Reflectometry (TDR) and capacitance sensors Measures soil dielectric constant (permittivity), related to volumetric moisture content Advantages: Very accurate No soil specific calibration for TDR Measure mulitple soil depths Easily connected to dataloggers Disadvantages: Can be costly (up to several thousand dollars) Measure very small soil volume Soil specific calibration may be needed Source: Muños-Carpena, 2004 (UF/IFAS)
Measuring Plant Water Status The Pressure Bomb Source: http://fruitsandnuts.ucdavis.edu/pressure_chamber/ Advantages: Measures plant water potential directly Disadvantages: Very labor intensive
Soil Water Content Where is water needed? The active pecan rooting zone is ~ 1 meter 20 cm 30 cm 50 cm Irrigations 90 cm 150 cm
Yield (kg kernel/cm 2 ) Yield (kg kernel/cm 2 ) Pecans don t like to be wet: good drainage is key for pecan production Y = (0.04*sand) + 0.79 R 2 = 0.57 Percent Sand Y = (-0.08*clay) + 4.16 R 2 = 0.58 Percent Clay
Basic Kinds of Irrigation Systems 1. Gravity Uses soil surface to convey and distribute water Oldest and still widely used Works best on flat terrain 2. Pressurized
Pressurized Distribution Micro-sprinklers Sprinklers Drip
Irrigation Efficiency Irrigation Efficiency = water needed water applied 100 Water Needed is determined by crop needs soil needs (e.g. leaching) environmental needs (frost protection, crop cooling, etc.)
All irrigation systems have inefficiencies Surface System Efficiency (%) Level Basin (flood) 60-80% Graded Border (flood) 55-75% Fixed Solid Set Sprinklers 70-85% Micro-sprinklers 85-95% Surface Drip 85-90% Sub-surface Drip 85-90% SOURCE: National Center for Appropriate Technology
Gravity Irrigation Advantages Low capital and operating cost Low or no energy requirement Simple inexpensive control structures Dirty water OK Good for salinity management Disadvantages Low irrigation efficiencies High labor requirements/no automation Negatively impacted by variable soil textures Fields must be well-graded and maintained Small irrigations not practical Vegetation, floating trash causes problems
Micro-sprinkler Irrigation Advantages High irrigation efficiency & uniformity Low labor requirements Easily automated Simple operations Can apply small irrigations Good for fertigation, leaching Disadvantages Higher capital costs System maintenance Higher evaporation losses Will damage contacted foliage
Drip Irrigation Advantages High irrigation efficiency Low labor requirements Easily automated Simple operations Can apply small irrigations Good for fertigation; nutrients placed close to roots Directed water distribution Low evaporative losses Disadvantages High capital costs System maintenance Leaks are difficult to detect (with buried drip) Plugging - requires clean filtered water, water chemistry monitoring required Not efficient for leaching, especially buried drip
Soil Depth (cm) Roots (g/2356 cm 3 soil) Drip lines were buried 35 cm deep 4 drip lines per row 2.00 and 3.50 m from the tree row Rows were 13 m apart Root Distribution with Buried Drip Irrigation 10 8 6 4 2 0 0 50 100 150 200 250 300 350 400 450 500 550 600 650 Distance from Trunk (cm) 90 to 120 60 to 90 30 to 60 0 to 30
Inter-row area is not watered Reduces water consumption Controls weeds but leaks are difficult to locate and repair
Where do water losses occur? Evaporation smaller sprinkler droplets greater losses Runoff Deep Percolation water moves below the root zone critical for salt management
Irrigation water is the major source of salts in most irrigated soils Annual salt addition Salt in irrigation water Salt added to orchard (ppm) (ds/m) (mtons/ha) 200 0.3 2.4 400 0.6 4.8 600 0.9 7.3 800 1.3 9.7 1000 1.6 12.1 1200 1.9 14.5 1400 2.2 16.9 1600 2.5 19.4 1800 2.8 21.8 2000 3.1 24.2 Assumes 120 cm of water per year
Salts A soluble salt is a molecule that dissolves in water, and separates into a cation (positively charged molecule) and an anion (negatively charged molecule) in solution. Sodium chloride is a familiar example: NaCl Na + Cl -
Soil Salts Common soluble cations in non-acidic soils: Common soluble anions in non-acidic soils: Chloride Cl - Sulfate SO 4 2- Bicarbonate HCO 3 - Carbonate CO 3 2- Nitrate NO 3 - Borate H 4 BO 4 - Calcium Ca 2+ Magnesium Mg 2+ Sodium Na + Potassium K + Ammonium NH = 4
Salinity Measurement: Electrical Conductivity Ions dissolved in water conduct electricity, so the total amount of soluble soil ions can be estimated by measuring the electrical conductivity (EC) of a soil water extract. EC e is measured on a saturated paste Alternative soil:water ratios include EC 1:1, EC 1:2, EC 1:5 Units: deci-siemens per meter (ds/m) EC x 640 ppm or mg/kg K + Ca 2+ Na + Mg 2+
Formation of soil salinity Salts added to the soil in irrigation water, fertilizers accumulate in the soil profile unless they are leached below the root zone. Poor drainage due to Inadequate Water Compacted Layers Heavy Soils Sodium-impacted soils
Effect of salts on plants When soluble salts dissolve in water, and anions and cations separate, they become hydrated This lowers the osmotic potential energy of soil water because the water is tied up by the anions and cations www.biology.arizona.edu/biochemistry/tutorials/chesmistry/graphics/nacl2.gif
Effect of Salts on Plants Water is drawn away from regions of low salt concentration and towards regions of high salt concentration To absorb water, the plant must overcome this salt gradient It is more difficult for a plant to absorb salty water than clean water outside root (soil solution) inside plant Root cell membranes allow water, but not salts, to move in and out of the cells
Symptoms of salt damage include decreased growth, marginal leaf necrosis, and eventual necrosis of entire leaves, and defoliation
Effects of soil salinity on various crop plants has been determined and may be fitted to a linear-plateau model The threshold level is the lowest soil salinity level that adversely affects plant performance Threshold
Salinity Impact on trees No problem Increasing problem Severe problem Soil EC e (ds/m) < 2 2-4 > 4 The threshold where tree growth begins to decline is approximately 2 ds/m Miyamoto 1986. Effects of saline water irrigation on soil salinity, pecan tree growth and nut production
Leaves with > 10% injury (% by wt) Dry Leaf Weight (g/seedling) Salinity tolerance varies among pecan root stalks, but we lack data for most cultivars APACHE BURKETT RIVERSIDE 0 2 4 6 8 10 12 14 Soil salinity (ds/m) 0 2 4 6 8 10 12 14 Soil salinity (ds/m) Miyamoto, 1985. Salt effects on seedling growth and ion uptake of 3 pecan rootstock cultivars
Irrigation Water Salinity Guidelines for Pecans Soil Texture EC (ds/m) Salinity limit mg/l (ppm) Clay, clay loam < 1.0 < 640 Loam 1.0 2.0 640-1280 Sand, loamy sand 2.0 2.5 1280 1600 Salinity tolerance is lower in clayey soils than in sandier soils because it is more difficult to leach salts from clay soils from SA Miyamoto
Y = 100 b(ec-a) where Y = relative yield at a given EC b = % yield decrease per ds/m EC = EC of soil A = threshold EC (where yield starts declining) Pecans have a salinity threshold of ~ 2.0 ds/m Growth decreases about 14% per ds/m above the threshold So the potential yield in soil with 3 ds/m (relative to a nonsalty soil) would be: Y = 100 14(3.0-2.0) = 86%
Leaching lowers soil profile salinity by carrying excess salts below the root zone. Na + SO = 4 K + H 2 O and Salts Cl - K + SO = Cl - 4 H 2 O Na +
Pore Water Solute Concentration 40% 60% 80%
Salt movement with various types of irrigation
Soil EC e (ds/m) after 90 days of daily irrigation (broccoli) Pecans with subsurface drip irrigation Salts accumulate above the drip emitters Leaching is problematic 0 1 2 3 4 5 6
Leaching Requirement (LR) The percentage of water (rain + irrigation) applied that must move below the root zone to control salt buildup. Equation for estimating LR: Leaching Requirement = EC w 5 EC w EC w EC w = water salinity (ds/m) EC e = threshold root zone saturated paste soil salinity (ds/m)
Leaching Requirement Leaching to Maintain Low Soil Salts The leaching requirement (LR) is the excess water (beyond tree needs) that must be applied to keep salts at a level that will not reduce yield LR increases as irrigation water salinity increases LR increases as soil clay content increases 120% 100% 80% 60% 40% Clay loam Loam Sand LR for 1000 ppm water Soil texture LR clay, clay loam 45% loam 19 45% sand, loamy sand 14 19% 20% 0% 0 200 400 600 800 1000 1200 1400 1600 Irrigation water EC