Water Gardens

The goal of this lesson is to simulate the effect of rainfall on different soil types and land uses. Runoff is what occurs when rain is not absorbed by the ground on which it falls and so then flows downhill. Eventually runoff water will drain into a stream or river. All of the land whose runoff water flows into a particular river or stream is known as that stream's watershed.

Why is it important to model runoff water?

Have you ever been to a construction site? Have you noticed the pathways that water carves through the soil? Those small ditches were caused by runoff water. Runoff water is the main mechanism of erosion in many areas. But this is not the only problem caused by runoff water. The topsoil, picked up by the water, is deposited into the river. This can cause serious water quality problems.

By making models, we can see what would occur in a system if we changed certain characteristics of it, without actually making the changes. For example, say that we wanted to know what would happen if we put in a parking lot in a place that is currently a field. We could simulate this in the model without actually putting in the parking lot.

How is surface water runoff determined?

There are a number of ways to determine the amount, or quantity, of water that runs off of a surface. Every time it rains a scientist could collect data on the runoff from the surface. After measuring the runoff from many different kinds of surfaces the scientist looks fro a mathematical way to predict the runoff.

We can also use computer models and simulations to estimate runoff. One of these methods is the Soil Conservation Service Runoff Curve Number (CN) method. The CN method is currently the most appropriate and authentic of the numerical models in use by soil scientists. The underlying mathematics of the SCS CN method are described in the runoff algorithm page. Some of the key concepts of the model are described here.

Over the years, data collected in the field has been analyzed. This data has been produced in a number of forms. One is graphically as an "X-Y" graph, shown below. On the "x" axis is the independent variable, in this case the amount of rainfall in inches. On the "y" axis is the dependent variable, the amount of direct runoff of water in inches.

The graph shows generally that as the rainfall increases, the amount of water that runs off increases. Makes sense, doesn't it? We use the data collected and the mathematics created from this data to build a predictive model. A predictive model is one that allows us to predict how much runoff will occur when the next big rainfall happens, such as if another hurricane strikes.

You should have noticed that there are a number of different curves on the graph. Each curve rises as the rainfall increases. Each of these curves represents what happens at a particular curve number, or CN. The CN comes from a table of data depending on what kind of area (city or farm), what kind of soil we have, and what is covering the soil. The numbers in the legend on the right of the graph represent the curve numbers, beginning at 40 and going to 100.

The amount of runoff in inches depends on the amount of rainfall in inches and the "curve number", or CN. The model can calculate the curve number if you give it 3 values.

What is a hydrologic soil group?

Soil groups are classified according to their drainage potential. Group A soils absorb a lot of water and are deep, well-drained, and composed of sand or gravel. Conversely, Group D soils do not absorb as much water and have a high run-off potential. Group D soils have a layer of high clay content near the surface or are shallow soils over bedrock or other material which does not absorb water. (For more info, visit the Penn State Earth System Science Center Database page on HSG percent

What are the different types of ground cover?

We can say that different types of communities often have different cover types. For example, an urban area, like a city, might have these cover types :

• open space (lawns, golf courses, cemeteries, parks, covered with grass)
• streets and roads
• paved parking lots, shopping centers
• houses and residential areas
• offices and business areas

There exists a complete set of curve numbers, or "CNs", for urban areas. Likewise, there is a chart of data for cultivated agricultural land, or farm land that is being used. Some example cover types are:

• fallow (bare soil or temporarily cover crops like grass)
• row crops (corn, potatoes)
• full grain crops (wheat, barley)

Finally, there are curve numbers for other agricultural lands:

• pasture, grassland, grazing land
• meadows
• brush
• woods/grass combinations
• woods

Hydrologic condition refers to how sparse or dense the ground cover is, and thus how easily water runs off the surface. If a pasture is overgrazed, there is only a little bit of grass. A well managed pasture has a dense stand of grass. "Sparse" and "dense" are meaningless or irrelevant for some types, such as pavement, but are significant qualifications on agricultural cover. The hydrologic condition is called "poor" if the ground cover is relatively sparse, "good" if it is relatively dense, and "fair" in between. Lands in poor hydrologic condition are more likely to have runoff.

How much is good? It varies, for different types of ground cover. For brush or pasture, over 75% is good and below 50% is poor. For open space in urban areas, above 70% is good. Residential urban areas also have a relevant hydrologic condition based on density of residences; town-houses and quarter-acre lots are considered poor, while large lots of a few acres are considered good. A full list is in "THE EXPLANATION OF TERMS".

The curve number data is provided to scientists in a table form. An example table, this one for other agricultural lands, is shown "IN A SEPARATE WINDOW". In the model itself, you have to "dial in" the cover type, the hydrologic condition, and the soil group (A,B,C,D). Based on your selections, the model looks up the correct curve number (CN), and puts that number into the mathematical model.

1. introduction

3. raingarden