Field Hydrologic Cycle

Chapter 6

Field hydrologic cycle

InfiltrationRunoffEvaporationTranspirationPercolationCapillary Rise

Drainage and Irrigation

Global Hydrologic Cycle

Radiant energy drives it and a lot of water is moved about annually.

Field Hydrologic Cycle This is the focus here--movement at a place--and these are the paths.

(any precipitation)

Infiltration

Water on soil may do what?

Principally, either infiltrate the soil surface or runoff from it.Of course, some of the water on the soil surface evaporatesbefore it has time to infiltrate or runoff. Ignore this.

Also, there is a proportion of any rain that is intercepted bycanopy or surface residue that does not reach the soil surfaceand evaporates.

i = intensity of rain (cm / h)

KS = saturated conductivity (cm / h)

If

i KS, what happens?

i > KS, what happens?

If the intensity of rainfall is less thanthe saturated hydraulic conductivity,all of the water that reaches the soilsurface should infiltrate it. On the other hand if the intensity is greater,it would seem that the difference, i – Ks would runoff. But not so. Itdepends.

See here. So why can there be behavior like in orange?

Recall Darcy’s Law for saturated flow,

Q = KS A ([D + L] / L)

This form is right for flow through a depth of porous material that is openTo the atmosphere at the bottom and has water on the upper surface butIt is a simplified form. The complete form would be,

Q = KS A ([pressure @ top + elevation @ top] – [pressure @ bottom + elevation @ bottom]) /

(elevation @ top – elevation @ bottom)

The denominator is length of flow and the numerator is the difference intotal water potential (pressure + gravitational) at the top and bottom, i.e.,the hydraulic gradient the is causing water flow. The simplified form comesabout because the pressure at the bottom is atmospheric (0), the elevation atthe bottom is the reference point (0), the pressure at the top is the depth ofwater (D) and the gravitational potential at the top is L relative to the bottom.

Now, let’s apply this way of thinking to a long column of soil that is unsaturatedbut that has become saturated just a the very surface due to rain at i > Ks.

Take the reference point at 1 cm below the surface. There the soil is still unsaturated and the water is under tension, say a – 100 cm (recall that whenwater potential is expressed as energy per weight, the unit is length). At the surface there is neither a positive pressure potential nor tension, i.e., the pressure (or tension) is 0. Writing Darcy’s Law for this situation,

Q = Ks A ([0 + 1] – [-100 + 0]) / (1 – 0) = Ks A (101 / 1)

Divide this by A to give flux, q, which has the units of depth per time,

q = Ks 101.

This is the maximum possible infiltration rate under these conditions. If the intensity of rain was > q = Ks 101, there would be some runoff but if theintensity was only > Ks, clearly there would not be runoff, just infiltration.

However, as infiltrating water moves deeper into the soil, the water content at 1 cm increases and the tension at that point decreases, i.e., becomes lessnegative so q decreases. As infiltration proceeds, there will come a timewhen q decreases to i and thereafter q < i so that runoff occurs.

When the soil at 1 cm is saturated, q = Ks. So the orange curve makes sense. Red one, too.

If the soil is already wet when it startsraining will runoff begin sooner than if thesoil had been drier?

Besides rain intensity

Initial soil moistureCrustSlope

Controls the soil water tension below the surface, so the answer to the belowquestion is obviously, yes.

This has to do withthe hydraulic conduc-tivity right at the surface.

Crust formation

Raindrops destroy aggregates Detached particles clog pores

So the hydraulic conductivity decreases, typically, a lot.Consequently, more runoff, no?

This old study illustrates the importance of protecting the soil surface fromthe effect of raindrop impact destroying surface aggregates and creating acrust. Even with i < initial Ks, a decrease in Ks results in runoff.

Runoff

Slope

Via transpiration soil water passes throughplant to atmosphere

Evaporates and leaves leaves

Water goes down gradient

Soil Root Stem Leaves Air

Evapotranspiration (ET)

Evaporation (E) and transpiration (T)

Both are evaporative processes,just different paths. So, oftenthe two a discussed jointly.

Factors affecting E and T

Radiant energy ET or

Vapor pressure ET

Temperature ET

Wind speed ET

Soil water ET

For the direction ofarrows shown, ET increases. Make sense?

By vapor pressure, what is meant is water vaporpressure in the air, i.e.,humidity.

Evaporation

Curves look a lot like infiltration curves exceptthey don’t approach a positive minimum, zeroinstead. The explanation is very similar to thatfor decrease in infiltration rate. See next slide.

Imagine a uniformly wet, say saturated, soil at time zero when evaporationbegins. Write Darcy’s Law for the upper 1 cm like before in terms of matric(tension) and gravitational potential,

q = K* ([-100 + 1] – [0 + 0]) / 1 = -K* 99

Here the flux, q, is not possible maximum, like with infiltration but real, and driven by radiant energy, wind speed, etc. (the external evaporativity). It won’ttake long before very surface of the soil has lost water and the water thereis under tension, say -100 cm. Also, the conductivity at the surface, K*, is lessthan the saturated conductivity. Notice the flux is negative, i.e., up, not down.

Now, let this surface drying continue so that the tension in water at the surface increases. Also, let the upward movement of water from below the surface continue so that at 1 cm the soil dries and tension increases there. Forthe sake of argument, call the tension at the surface –2000 cm and thetension at 1 cm –1990 cm. At this point, the conductivity is a lot less, K**, so

q = -K** 9

which is very most likely considerably less than the external evaporativity.

How do you control evaporation?

Mulches

Conservation tillage

How do these work?

Reduce

Solar radiation inputSoil temperatureVapor pressure gradientAir currents

How do you control transpiration?

Kill weeds

PercolationRunoff

Runoff rainfall > infiltration

Percolation infiltration > ET

Would you expect more percolation in thewinter or in the summer?

OK, runoff occurs when rainfall > infiltration. So, too,downward movement of water in the soil (percolation)occurs when infiltration > ET.

Percolation and Groundwater

Soil surface Vadose zone

Water table################################## Groundwater$$$$$$$$$$$$$$$$$ $$$$$$$$$$$$$$$$$ Impervious layer

This is supposed to represent groundwater morphology, a saturated zone abovean impervious layer and an unsaturated zone about this (called vadose zone).Arrows show percolation, recharging the saturated zone.

Just above water table is a saturated zone atless than air pressure

Capillary fringe

Height depends on largest pore size

Due to capillarity, pores canremain filled with water upto a certain tension – the smaller the pores, the greater(more negative pressure) thistension.

However, bore a well and water willfill it to depth of the free water surface, i.e., at atmospheric pressure, which is lower than the depth at which the soilis saturated.

Upward movementreplenishes soil water

What’s a potentialproblem here?

This movement is due to a gradient inmatric potential (tension) that exceedsthe downward gradient in gravitationalpotential.

Answer

Movement of dissolved substances

Loss of plant nutrientsWater contamination

Land Drainage

Surface and subsurface

AerationSoil warms fasterQuicker access for operations

The reason for drainage is to improve aeration, however,there are these other benefits pertaining to planting and operations.

Surface

Faster removal of ponded water

Fill depressions and build ditches

Subsurface

Pipes or ditches to lower water table

Curious phenomenon, radial flow to drains. The streamlines shown bythe dye are perpendicular to the steepest gradient in water potential.

So, would it take more closely spaced drain lines to lower a shallow water table in a sand or in a clay and why so?

Irrigation

SurfaceSprinklerTrickle

Surface

InexpensiveOK for nearly level fieldsUniformity and efficiency poor

Sprinkler

ExpensiveMore uniform and better efficiency

Trickle or drip

Expensive and difficult to maintain

Appropriate where water is scarceHigh efficiency