Lecture 7 b Soil Water – Part 2 Source: Dept of Agriculture Bulletin 462, 1960

Water Movement Movie University of Arizona Be prepared for exam questions from this movie!

Describe in your own words what happens to the water in the diagram below. Water Soil A horizon - Air Dry

Answer The water moves sideways and downward at the same rate. This is because of adhesion and cohesion. Would the movement be different if the soil was saturated?  Yes. The movement would mainly be downward due to gravity. WATER

Water Movement Water Loam Sand

Water Movement Water front does not move into sand until loam is saturated Water Loam Sand t 1 t 2 t3 t4

Water Movement Water front moves into clay upon contact with clay, but because it moves slow water builds up above the clay layer. Water loam clay

Summary Points from Water Movement Movie University of Arizona 1) Pore size is one of the most important fundamental properties affecting how water moves through soil. Larger pores as in sand conduct water more rapidly than smaller pores in clay. 2) The two forces that allow water to move through soil are gravitational forces and capillary forces. Capillary forces are greater in small pores than in large pores.

3) Gravitational and capillary forces act simultaneously in soils. Capillary action involves two types of attractions, adhesion and cohesion. Adhesion is attraction of water molecules to solid surfaces; cohesion is the attraction of water molecules to each other. Gravity pulls water downward when the water is not held by capillary action. Thus gravity influences water in saturated soils. 4) Sandy soils contain larger pores than clay soils, but do not contain as much total pore space.

5) Sandy soils do not contain as much water per unit volume of soil as clay soils. 6) Factors that affect water movement through soil include texture, structure, organic matter and bulk density. Any condition that affects soil pore size and shape will affect water movement. 7) Examples include compaction, tillage, decayed root channels and worm holes. 8) The rate and direction of water moving through soil is also affected by soil layers of different material. Abrupt changes in pore size from one layer to the next affect water movement. When fine soil overlies coarse soil, downward water movement will temporally stop at the fine coarse interface until the fine layer above the interface is nearly saturation.

9) When a coarse soil is above a fine soil, the rapid water movement in the coarse soil is greater than through the clay and water will build up above the fine layer as the water front comes in contact with the fine layer. This can result in a build up of a perched water table if water continues to enter the coarse layer.

World Water Total 97.2 % Ocean 2.8 % Fresh  2.15 % glaciers  0.65 % ground water  % streams  % lakes  % seas  % soil  % atmosphere

Hydrologic Cycle is driven by the energy from the sun-Evaporation Water is heated by the sun Surface molecules become sufficiently energized to break free of the attractive force binding them together Water molecules evaporate and rise as invisible vapor into the atmosphere

Hydrologic Cycle -Transpiration Water vapor emitted from plant leaves Actively growing plants transpire 5 to 10 times as much water as they can hold at once These water particles then collect and form clouds

Hydrologic Cycle Evaporation Transpiration Soil Water Storage determines ground water recharge

Soil Water and Plant Use

Water Budget

Ap May June July Aug. Sept Oct R echarge R unoff Evapotranspiration Precipitation Soil moisture utilization Actual ET Potential ET Water amount ET > Precip = Soil moisture utilization Precip > ET = Recharge, surplus, and runoff Water Balance Diagram

Calculating Soil Moisture Gravimetric  The mass of water in a given mass of soil (kg of water per kg of soil). Pw = Percent water by weight or Pw = gr. water ÷ gr. OD soil Pw = (weight of wet soil – weight of oven dry soil) X 100 weight of oven dry soil

Calculating Soil Moisture Volumetric  The volume of water in a given volume of soil (m 3 of water per m 3 of soil)  Pv = Vol H20 ml ÷ Vol soil ml Pv = Percent volumetric P v = P w X bulk density

Calculating Soil Moisture Inches of water per depth of soil …. or how many inches of water are in a specified depth of soil. Inches water = Pv x (depth of soil) … or.. depth of soil wetted = (inches of water) ÷ Pv

What determines Plant Available Water Capacity (AWC) AWC = FC-WP Rooting depth a) type of plants, b) growing stage Depth of root limiting layers Infiltration vs. runoff (more water entering soil, more will be stored ) Amount of coarse fragments (gravel) Soil Texture - size and amount of pores silt loam has greatest AWC, followed by loam, clay loam silty clay loam

Soil Water Classification

AWC by Texture: UNITS PLIZ Texture Available Water Capacity in Inches/Foot of Depth Coarse Sands Fine Sands Loamy Sand Sandy Loams Fine Sandy Loam Loam Silt Loams Clay Loam Silty Clay Loams Silty Clay Clay DYAD= a soil with 2 feet of ls over 2 feet of silt loam has how many inches of AWC if all 4 feet is at field capacity?

Sample Problem: Gravimetric determination of soil water Wt. of cylinder + oven dry soil = 240g wt. cylinder at field capacity =350g wt cylinder at wilt point = 300 Wt cylinder on June 1 = 320 volume cylinder = 200 cc BD = 240/200 = 1.2 g/cc % water by wt. at FC = ( )/240x100 = 45.8% % water by vol at FC = ( )/200 x100 = 55% and %water by wt. X BD = % water by Vol Or 45.8 X 1.2 = 55% % water by vol at WP = /200 = 30%

AWC = FC - WP ( - 15) % water by vol at Field Capacity = %FC %water by vol at Wilt Point = % WP % FC - % WP = % AWC = 25% & ( % water x inch soil = inch water) For 4 feet of soil 25% AWC means that.25 x 48 inch. = 12 inches of water stored in 48 inches of soil. 0 4 ft. = 12 inches of water available/ 4 feet

Rainfall Infiltration: UNITS How deep will a 1 inch rainfall infiltrate the soil on June 1. Soil will be wet to field capacity than water moves deeper. And % water vol x soil depth = inches of water or Inches of soil = amount of water / %water vol % water by vol between June 1 & and FC = /200=.15 Or 1/.15 = 6.67 inches of soil is depth of wetting