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**Yhd-12.3105 Subsurface Hydrology**

Unsaturated Flow Teemu Kokkonen Tel Room: 272 (Tietotie 1 E) Water Engineering Department of Civil and Environmental Engineering Aalto University School of Engineering

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**Soil Moisture Profile – From Groundwater level to soil surface**

Recall some definitions Groundwater level is defined to be that level where soil water pressure is atmospheric Below the groundwater level the soil is saturated with water and above the groundwater level the soil is unsaturated Immediately above the groundwater level there is a capillary fringe that is (amost) fully saturated

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Unsaturated Zone Does the percolating water in the figure above enter the subsurface drain? Why / why not? Water originating from precipitation or irrigation infiltrates through the soil surface and percolates through the unsaturated zone This forms recharge to an aquifer Harmful substances move with water In unsaturated zone the water pressure is negative Water is retained in soil by capillary forces, which are a combination of cohesive and adhesive forces

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Tensiometer Negative water pressure in soil is measured using a tensiometer Soil sample Porous plate Water hc 1. How can you read the pressure head in the soil sample using the tensiometer shown in the figure? 2. The porous plate needs to be airtight. Why? 3. Why does the water entering the soil sample does not significantly affect the measurement?

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Water Retention Curve A graph that shows the relationship between soil water pressure head and moisture content of soil is called the water retention curve In the water retention curve the soil water pressure head is typically expressed as a pF value pF value is the 10-based logarithm of the absolute value of the pressure head expressed in centimeters of water column height As pressure head values range across a large scale taking a logarithm lead s to a garph that is easier to interpret Pressure head is – 100 cm. What is the corresponding pF value? Pressure head is – 100 cm => pF value is 2

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**Water Retention Curve III III: Residual moisture content qres qres**

II: Air-entry pressure head ha ha II I I: Porosity

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Water Retention Curve It will not be a great surprise that different soils have water retention curves of different shape Clay Sand Which one of the shown water retention curves is for a clay soil and which one for a sand soil? Why?

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**Water Flow in Unsaturated Zone**

What are the differences to saturated (groundwater) flow? Hydraulic conductivity is a function of the moisture content of soil When moisture content decreases large soil pores are emptied first, which leads both in reduced cross-sectional area of flow and increased tortuosity of the flow paths => hydraulic conductivity drops The air-filled pore space is a function of the moisture content of soil Recall the large difference (several orders of magnitude) in the storativity of confined and unconfined aquifers Recall that moisture content and pressure head are related via the water retention curve Hydraulic conductivity and the air-filled pore space can also be expressed as a function of pressure head

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**Darcy’s Law in Unsaturated Zone**

As presented earlier the hydraulic head H is the sum of pressure head h and gravity head z In the unsaturated flow the interest often is to study percolation to groundwater, so let us first write Darcy’s law in one dimension and in vertical direction Here the direction of the z-axis is points downward – hence the negative sign.

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**Darcy’s Law in Unsaturated Zone: 3D**

Why is the -1 present in the equation of qz missing from the equations for qx and qy?

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**Unsaturated Hydraulic Conductivity**

Coarse gravel Relationship between the pressure head and the hydraulic conductivity for different soil types Fine sand Peat Clay Hydraulic conductivity Pressure head

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**Unsaturated Hydraulic Conductivity**

The water retention curve (pF curve) and the unsaturated hydraulic conductivity can be described with the following equations originally proposed by M.Th. van Genuchten and Y. Mualem Where is the soil moisture (cm3/cm3), R is the residual water content of soil (cm3/cm3), S is the saturated water content of soil (cm3/cm3), S is the saturation of soil (cm3/cm3), h is the pressure head (cm), and ha is the air entry pressure head. Symbols , , and refer to the parameters of the van Genuchten model, and = 1 – 1/. K is the unsaturated hydraulic conductivity, KS is the saturated hydraulic conductivity (cm/h), and KR is the relative conductivity of unsaturated soil (KR = K / KS).

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**Reminder: Transient Groundwater Flow in 3D**

Specific storativity S0 volume of water added to storage, per unit volume and per unit rise in hydraulic head

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**Flow in Unsaturated Zone: Richards’ Equation**

Specific moisture capacity: Differential water capacity: Volume of water released from (or added to) storage per unit decrease (or increase) of pressure head C [1/m]

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**Differential Water Capacity**

The definition was: Differential water capacity: C [1/m] Volume of water released from (or added to) storage per unit decrease (or increase) of pressure head From the definition above it follows: ,where q is the volumetric moisture content So:

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**Differential Water Capacity**

Moisture content q Pressure head h Dq Dh

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**Numerical Solution – Richards Equation**

Let us discretize the Richards equation in 2D for a longitudial section: x (i) Sink / source z (j)

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**Numerical Solution – Richards Equation**

z (j) x (i) Dx Dz

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**Numerical Solution – Richards Equation**

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**Numerical Solution – Richards Equation**

Approximating the differential water capacity C Estimate using the Van Genuchten equation the moisture content that corresponds to the pressure head at the desired time and location Perturbate the pressure head with a small displacement of Dh Compute the moisture content at h + Dh Now you can estimate C using the difference method as Recall that How would you approximate C?

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