Soil water

The infiltration capacity and antecedent wetness conditions of soils are typically a primary control on how a watershed will respond to precipitation. This module will develop the logic of predicting the hydrologic behavior of soils during precipitation events at watershed scales based on the fundamentals of soil physics (3:46 min).

Contents of this module

Substantial water movement through soils tends to be episodic and driven by key thresholds. The wetness of soils can be categorized into three basic states delineated by these key thresholds that defining the availability of water to plants and the ability of water to drain from soils under the influence of gravity (4:27 min).

If we want to put a number on how much water is in the soil, hydrologists tend to use a volumetric perspective. We can quantify the amount of water by an absolute volume, but dimensionless fractions of the total volume tend to provide more useful metrics of pore space and water content for hydrologic applications (2:40 min).

The saturation state and the density of the soil are also important to understanding the hydrologic behavior of soils (5:05 min).

The gravimetric technique is one of the more robust ways to estimate the water content of smaller samples of soil. Working through the gravimetric technique and the dimensionality of the calculation also helps practice the appropriate use of the metrics of water content discussed so far.  The following is a detailed example of the calculations for the gravimetric technique as well as a brief introduction for a few other ways to estimate soil water content (14:52 min).

While we have talked in general about the ability of soil to retain water, we have not established the physics underlying the mechanism. If pores in soil can hold water against gravity, there must be some force that can oppose gravity to allow that retention to happen. This opposing force arises from molecular adhesion and cohesion is called capillary force, which is closely related to the tension that can be measured in the water retained in soil in unsaturated conditions (10:07 min).

If there is another force that affects water in unsaturated soils, then it only makes sense that there must be another form of potential energy associated with that force. Hence, we have to start considering another component of hydraulic head, or the pressure head (4:32 min).

Soil water under tension in unsaturated conditions will have a negative pressure head. Because this negative pressure head is a component of the total hydraulic head and hydraulic gradient, the ability to measure or estimate negative pressure heads is critical to predicting water movement in unsaturated conditions (7:51 min).

We finally have all the key concepts necessary to understand how hydrologists attempt to predict soil water movement. Before we get into the complexity of predicting flow or flux of water through soils, let's start with just predicting the direction of water movement based on the direction of calculated hydraulic gradients (8:10 min).

Let's be sure the logic of this example is clear by working through the math in the opposite direction (3:34 min).

Regardless of whether you are dealing with saturated or unsaturated conditions, Darcy's Law is by far the most commonly applied predictive tool for predicting the flux of water through a porous medium (5:15 min).

The application of Darcy's Law to unsaturated soils can be a bit less intuitive than applications to saturated conditions because you have to consider that both pressure head and hydraulic conductivity change dramatically with water content. While the relationship between water content and tension or hydraulic conductivity will change with different pore structures, having a general sense of the nature of these relationships is critical to building intuition regarding the infiltration of water through soils (7:50 min).

The retention of water in soils is a critical element of agricultural land management. Thinking through a simple example of why loams balance storage and plant availability of water to be more generally suitable for crop production (relative to clay or sand) helps solidify the concepts and consequences of tension we have been discussing (4:28 min).

Ultimately, a watershed hydrologists needs a strong grasp on these concepts because they are needed to understand the logic of how we typically predict the infiltration capacity of soils across a watershed. Infiltration capacity is in turn a dominant concept in predicting whether precipitation will be partitioned to storage or runoff during a given storm (4:00 min).

The combination of Darcy's Law with a conservation of mass equation yields Richards equation, which is by far the most applied model for predicting subsurface flow in unsaturated conditions. While solving Richards equation with confidence at watershed scales is problematic for many reasons, the basics of its derivation help to bring all the concepts discussed so far into an actual strategy for predicting water infiltration through unsaturated soils (8:59 min).

Because soil needs to have a fair amount of water in it before the water can easily move, predicting the behavior of infiltration of water during a given precipitation event requires understanding the behavior of wetting fronts and the consequences to infiltration capacity (6:29 min).

Green Ampt Theory provides an approach to predicting infiltration rates that is consistent with the fundamental theory of soil physics, but is simplified by considering the fundamental behavior of wetting fronts (13:36 min).

We have established the logic of Green Ampt Theory. Now let's see if the algebra of the Green Ampt equation supports our intuition about how infiltration rates decrease through a precipitation event and approach a minimum rate of the saturated hydraulic conductivity (12:12 min).

Another tricky example for building your intuition is to imagine how a wetting front moving through a finer textured soil will respond when it encounters a coarser textured soil. Here is a hint. Your intuition will fail you if you consider only the hydraulic conductivity as limiting water movement through the sediments and only the gradient in gravitational head as the driver of water movement (4:18 min).

With the mathematics and logic of the Green Ampt approach in place, let's consider the implications of predicting changes in infiltration capacity on predicting the partitioning of water between storage in soils and potential stream flow generation during a storm (7:47 min).

Finally, some good news and some bad news. The good news is that the Green Ampt approach provides a relatively simple approach that helps with effective prediction of watershed response to precipitation, even with a limited amount of data regarding the state of the watershed soils. The bad news is that theory based on water moving through homogenous porous substrate is technically almost never applicable in real watersheds. The theory is still applied and can be useful for effective prediction. But we use simplified models understanding that the models and metrics we employ are an effective representation of the system, rather than necessarily a direct physical representation of how water moves. A current hot topic in the hydrologic science community is the appropriate adaptation of theory necessary to more directly account for the effects of macroporosity. The details of this topic are outside the scope of this class but the general implications are important to recognize in understanding the limitations of simplified models to directly represent hydrologic phenomena (4:09 min).

Study guide

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Study guides are designed to summarize the vocabulary, concepts, and mathematics learned in this module.

Readings from Dingman (3rd ed)

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A list of associated readings from Physical Hydrology by S. Lawrence Dingman (3rd edition)

Slides used in videos

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Laboratory preparation materials for this module

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Time lapse videos of water movement in soil

Video from The Irrigation Toolbox with time lapse video of several examples of water movement through soils (31:24 min).

Video from Gembloux Agro-Bio Tech with time lapse video of several examples of water movement through soils (16:25 min).

Sign conventions in hydraulic gradients

An alternative application of sign conventions to hydraulic gradient to think about the direction of water movement along a hydraulic gradient in soils (4:21 min).