Irrigation and Runoff Management. Water Quality for Irrigation. Soil Salinity and Sodicity

Transcription

1 Irrigation and Runoff Management Water Quality for Irrigation Soil Salinity and Sodicity Dr. Sharon Benes, Fresno State Franz Niederholzer, UC Farm Advisor Irrigation and Runoff Management Water Quality for Irrigation Soil Salinity and Sodicity 1

2 How to measure soil moisture Soil water potential ( in -cbars) or its inverse, *tension (+cbars) or: Soil moisture content ( m or v ) Transpirational Pull moves water into plants and through leaves following the water pressure gradient from small to large pull. 2

3 Instruments that measure soil water potential ( ) / tension -- tensiometers -- porous block sensors gypsum blocks Watermarks) Tensiometers (soil tension, 0 ~100 cbars) Irrometer Co. Hortau Inc. 3

4 Porous block sensors, e.g. Watermark (soil tension, 0 - ~200 cbars) Instruments measuring volumetric water content ( v) Neutron probe (access tube, mobile) Capacitance probes -- No access tube: ECH20 -- Access tube, mobile: Diviner -- Access tube, instrument permanently installed, providing continuous monitoring (soil moisture profiles) Enviroscan, Crop Sense (T-tape Int l.) TDR and FDR 4

5 Neutron probe ( v) Enviroscan -- permanently installed -- continuous monitoring C-probe * w/ telemetry *Same as Enviroscan, but remote access to data 5

6 Pressure bomb measures plant water status. Irrigation Scheduling Soil moisture sensing and/or Evapotranspiration (ET) estimation, e.g. by CIMIS stations ET = E (evaporation from the soil surface) + T (transpiration (loss of water from plant) 6

7 Step 1. Irrigation Application Thresholds From Soil Tension Sampling Soil Tension Sand/loamy sand Sandy loam Loam/silt loam Clay loam/clay (entibars) Depletion of Plant Available Water (%) Not fully drained Not fully drained Step 2. How much water to apply? How deep is the root zone? How much water (acre-inch) is 50% of plant available water in each horizon (soil layer)? Add it up Correct for efficiency Apply needed water + efficiency correction 7

8 CIMIS Weather Station --reference ET (ETo) -- Micrometeorological data -- Temperature -- Wind speed -- Net radiation -- Rain gauge -- other Used to estimate the ET of a well-watered grass under standard conditions Reference (ET o ) from CIMIS Crop coefficient (Kc) specific to the crop & changes as crop cover/canopy s Crop ET (ET c ) estimated as: ET c = ET o x K c * Note: the most effective irrigation scheduling combines soil moisture measurement and calculations of water loss by ET 8

9 Crop coefficient (Kc) through season. How much water to apply? Et o for the week (or time considered) Kc for crop Et c =Et o X K c Correct for efficiency 9

10 Water quality for irrigation Soil Salinity and Sodicity 10

11 Management of Saline & Sodic Soils ANR# FAO #29 T0234E00.HTM Classification of Soils (or water) Salinity (ECe) (ds/m) Sodicity (SAR) ph Physical Condition of Soil Normal <4 <13 <8.5 Normal Saline >4 <13 <8.5 Normal Salinesodic >4 >13 <8.5 Normal Sodic <4 >13 >8.5 Poor 11

12 Soluble Salt Composition Cations: Sodium, calcium, magnesium Na + Ca 2+ Mg 2+ Anions: Chloride, sulfate, bicarbonate Cl - SO 4 2- HCO 3 - Plus: smaller quantities of potassium & ammonium (cations) & nitrate & carbonate (anions) Commonly analyzed ionic constituents in irrigation water and conversion factor (PPM to meq/l). Symbol PPM / meq per liter* Cations calcium Ca 20 magnesium Mg 12 sodium Na 23 potassium K 39 iron Fe ** manganese Mn ** Anions chloride Cl 35 sulfate SO 4 48 sulfate-sulfur SO 4 -S 16 bicarbonate HCO 3 61 carbonate CO 3 30 nitrate NO 3 62 nitrate-nitrogen NO 3 -N 14 boron B ** 12

13 Sources of Soluble Salts Soil minerals (marine parent material) Irrigation water Fertilizers & Soil Amendments Shallow ground water Climate Low precip and high temps (precip/evap. < 1) Evaporation moves water (& salts) to soil surface, esp. in the presence of shallow GW Texture Westside San Joaquin Valley Soils: Marine sediments (salts, B, Se) 13

14 Net import of salt to westside SJV in irrigation water = 40 railroad cars daily Effect on Plant Growth Osmotic effect Salts lower the soil water potential more difficult for plants to extract water from the soil. ~Immediate Specific ion effect (Na + or Cl - toxicity) Accumulation over time foliar injury. Less toxicity with calcium and sulfate salts 14

15 Salinity effects on plants: osmotic Salinity effects on plants: specific ion (Na +, Cl - ) 15

16 Management options to impacts of soil salinity Choose more salt tolerant crop (Maas Hoffman tables) Manipulate planting position -- plant on edges of bed for furrow-irrigated -- plant along drip line for drip-irrigated. -- Salinity is lower near the water source Reclaim the soil (leaching) Maas Hoffman Salinity Tolerance Tables 16

17 Changing planting position to salinity impact Raised bed, furrow-irrigated: plant on shoulders of bed no yes Sprinkler irrigation Leaching: -- Apply extra irrigation water to push salts below the root zone, e.g. pre-plant -- Winter rains 17

18 Pre-plant leaching in the Imperial Valley Leaching Requirement (LR) LR = minimum amount of water, in excess of irrigation requirement, needed to to leach salts through a water-saturated soil and ensure proper salt balance. Expressed as the fraction* of the water needed to wet the soil (bring to FC) that must be applied additionally. *So LR is a number between 0 and 1. 18

19 Leaching Requirement LR = EC w X 100 Where: (5 X EC e *) EC w ECw = irrigation water salinity ECe = soil salinity that your crop can tolerate (from MH salinity tolerance tables) Example: irrigating with 1.4 ds/m water, tomato crop requires 36 in. of water, threshold soil salinty (ECe) from MH tables is 2.5 ds/m 1) LR = EC w X 100 (5 X EC e ) EC w = 1.4 X 100 = (5 X 2.5) ) Total water needed (AW): AW = ET 1 LR = 36 in. = 41 in. (1040 mm)

20 Colloquial term: alkali Sodicity Problem: sodium disperses clays! Degrades soil structure Dispersed clay particles move with water into soil macropores and clog them. Water penetration slows & sealing (crusting) of soil surface Water ponds; cannot leach to a sufficient depth Poor aeration affects root growth & uptake Sodium Dispersion of Clays Ca 2+ (or O.M.) Na + Calcium (left) overcomes repulsion between negative charges on clays, aggregrating them and building soil structure. Weakly attracted sodium ions (Na+) enlarge the distance between colloids and repulsive forces keep clay particles dispersed. 20

21 SAR indicates the hazard that soil colloids will disperse. Likelihood of infiltration problems --High SAR greater hazard For a given SAR, dispersion will be greater as the ph s (and less as salinity s) SAR* = [Na+] * *Concs. are in meq/l [Ca ++ + Mg ++ ] * 2 SAR vs. ESP ESP = Exchangeable Sodium Percentage (Ex Na/CEC)*100 Critical value for definition of sodic conditions = 15 ESP used in calculation of Gypsum requirement SAR = Sodium Adsorption Ratio Critical value for definition of sodic conditions = 13 Used in Ayers & Westcott (FAO 29) infiltration hazard determination (graph in next slides) 21

22 Reclaiming saline-sodic & sodic Soils If SAR > 10-13, soil has excess sodium and susceptible to clay dispersion poor infiltration 1) Establish internal drainage. 2) Replace excess exchangeable sodium with calcium -- gypsum most commonly used -- can use soil sulfur, if free lime (CaCO 3 ) present in soil 3) Leach the displaced sodium. Soil Amendments Inorganic - adjust soil ph or chemistry Lime raise soil ph (for acidic soils) Sulfur lower soil ph (for alkaline soils) or to liberate native Ca Gypsum improve structure/infiltration (sodic soils, or low Ca soils) Organic add nutrients, build soil structure Compost Manure 22

23 Irrigation Water Quality Ayers & Westcot, FAO Agricultural & Drainage Paper #29 Irrigation water constituents that pose a clogging risk in irrigation systems Potential problem. Physical suspended solids (clay, etc.) Units None Degree of restriction on use Slight to moderate Severe PPM < > 100 Chemical iron PPM < > 1.5 manganese PPM < > 1.5 bicarbonate meq/liter < >

24 Water Quality Guidelines (Salinity & Sodicity) Irrigation water quality guidelines regarding sodium and chloride concentration; values are PPM (= mg/l). Specific ion Sodium No restriction Degree of restriction on use Slight or moderate Trees, vines and other sensitive crops Severe Surface irrigation < > 200 Sprinkler irrigation < 70 < 70 Vegetables Sprinkler irrigation < > 460 Chloride Trees, vines and other sensitive crops Surface irrigation < > 350 Sprinkler irrigation < 100 > 100 Vegetables Sprinkler irrigation < >

25 SAR and salinity (EC w ) influence the infiltrability of water Irrigation water SAR and EC influence infiltration. Likelihood of water infiltration problems Irrigation water SAR unlikely when EC w is more than likely when EC w is less than

26 Water Quality Guidelines (Toxic Ion) 26