Nutrients from the Researcher Perspective (2-D Processes)
In InterWET, the nutrients from the researcher perspective gives a two
dimensional view or process level view of nutrients using the Loading Functions
from the GWLF Model.
1. Nutrient Calculators.
The nutrient calculators are the most complex of the calculators used in the researcher perspective of InterWET. There are three different calculators for nutrients, corresponding to a dry year, an average year, and a wet weather year. To start, click the button below to load only the Nutrient Calculator for an average year.
The calculator can be resized to fit your screen.
The nutrient calculators use a combination of factors from the previous calculators and a new waste water factor. Dissolved nutrients are strongly tied to factors which affect runoff and stream flow. Therefore, land cover, soils, and the groundwater aquifer, called groundwater discharge rate in the groundwater flow calculator, will affect both stream flow and dissolved nutrients. Likewise, sediment-bound nutrients are strongly tied to factors which affect erosion. Therefore, land cover, soils, slope, and slope length will affect both erosion and sediment-bound nutrient. The new factor is waste water disposal. Waste water from homes can be treated in on-site septic systems, in regional sewage treatment plants, or directly discharged to streams. Each treatment method will release different levels of dissolved nutrients to streams.
The nutrient calculators are based in annual estimates. Certain factors from previous calculators do not influence the annual estimates. For example, the amount of precipitation used in the runoff calculator is not need since the nutrient calculators use one year of daily precipitation values to calculate predictions. Likewise, the storm frequency factor from the sediment calculator is also not needed.
2.Effects of Annual Weather
In the average year nutrient calculator, set the Land Cover to Pasture, Soil Texture to Silt & Sand, Groundwater Aquifer to Sandstone, Slope to Moderate, Length along Slope to Long, and Waste Water to All normal septic tanks. When you click calculate, a window should pop up telling you this calculation may take a few minutes. While the calculator is determining the values for the outputs, the box below the Calculate and Clear buttons will read "Calculating." When the calculator is finished, this box will read "Complete" and each output box should contain a number.
Notice the units for the various outputs. The runoff is listed as a percent of the stream flow. The implies that the remaining stream flow comes from groundwater flow. The stream flow is listed as a percent of the precipitation. Remember that the remaining precipitation went to either evapotranspiration, groundwater storage, or snow pack storage. The erosion or detached sediment has units of tons per acre. The nutrient estimates are listed in units of lbs per acre. These units keep the nutrient estimates closely tied to their sources and influencing factors. In the conservationist perspective, we will see that monitoring of nutrients uses units like mg per liter, which indicates both the amount of nutrient (mg) and the stream flow (l). For easier understanding of the processes in the researcher perspective, the units were kept at lbs per acre.
Now notice the values of each output, especially the nutrients. The introduction to nutrients mentions that usually one is concerned with dissolved nitrogen, especially dissolved nitrate, and sediment-bound phosphorus. Dissolved nitrogen and sediment-bound phosphorus are both higher than their other nutrient components.
Now load the dry and wet year calculators and use the same set of inputs and compare the outputs.
Click the button below to load the Nutrient Calculator for a dry year.
Click the button below to load the Nutrient Calculator for a wet year.
In order to view all of the outputs at the same time, you can resize the calculators smaller so a scroll bar appears on the right. Scroll down to just show the outputs. Notice how the dissolved nutrient levels increase as the annual precipitation increases from dry to average to wet. Also notice how the stream flow increases. Since stream flow is listed as percentage of precipitation, the actual flow in the streams will increase dramatically as annual rainfall increases, as expected.
Also notice the response of the sediment-bound nutrients. Their levels are not tied to the increase in precipitation, but to the annual weather that caused the most erosion. For this case, the average precipitation year had more erosion than the wet year. Remember that the erosion is tied to rainfall energy. While the wet year had a higher total precipitation, the average year had a higher total precipitation energy. This could be caused by a small number of large, very infrequent storms during the wet year.
3.Effects of Groundwater Aquifer
Using the average year nutrient calculator, experiment with the different types of groundwater aquifers.
You should notice changing the aquifers only affects the dissolved nutrients, not the sediment-bound nutrients. The aquifers with higher discharge rates caused slightly higher dissolved nutrient levels.
4.Effects of Slope and Length along the Slope
Again using the average year calculator, experiment with different slope and lengths along the slope. Since these two factors only affect erosion and consequently the sediment-bound nutrients, one would only expect these outputs to change. You should find that steeper, longer areas produce more erosion and increase the sediment-bound nutrient levels, but does not affect the dissolved nutrients.
5.Effects of Land Cover and Soil Texture
Changing the levels of Land Cover again shows the dependence between dissolved nutrients and runoff and between erosion and sediment-bound nutrients. Experiment with the different Land Covers. Notice how dissolved nutrients go up when the runoff increases and sediment-bound nutrients go up when erosion increases.
Also experiment with different soil textures. Notice how the soils which produce less runoff also produce lower dissolved nutrients levels. Also remember from the erosion calculator how the most erosion came from silty clay loam, which is Clay & Slit in this calculator.
6.Effects of Waste Water Disposal
The final factor influencing nutrient levels is the method of waste water disposal. Waste water only affects the dissolved nutrient levels. Experiment with the different waste water options to see the impacts from the different disposal methods.
The options consider sewage that goes through sewage treatment plants, normal and failing septic systems, and direct discharge. For dissolved nitrogen, treated sewage is lowest, then septic, and direct discharge. Dissolved phosphorus, on the other hand, is lowest for septic, then direct discharge and treated are about the same. Dissolved phosphorus is absorbed by the soil and plants for septic systems, so these levels are lower. Even though direct discharge and treated have high levels, the dissolved phosphorus levels are still extremely low. The biggest nutrient concern with waste water is dissolved nitrogen.
Summary
1. Dissolved nutrient levels are closely tied to runoff and stream flow. Consequently, those factors which increase runoff and stream flow will also increase dissolved nutrients. These factors include wetter weather, faster discharging aquifers, and land covers and soil which produce more runoff.
2. Sediment-bound nutrient levels are closely tied to erosion or detached sediment. Therefore, those factors which increase erosion will increase sediment-bound nutrients. These factors include higher slopes and slope lengths and land covers and soils that allow more soil detachment. An increase in annual precipitation weather may sometimes also increase the erosion and sediment-bound nutrient levels.
3. Waste water disposal methods will impact dissolved nutrient levels. Sewage treatment plants produce less dissolved nitrogen, but more dissolved phosphorus than septic tanks. Direct discharge produces the most dissolved nitrogen, but about the same level of dissolved phosphorus as sewage treatment plants.
More Details?
Check out the on-line dissertation on InterWET for more information behind the theory and techniques used to make this calculator.
Questions or Problems? Let me know at parson@andassoc.com.
Last revision 9/9/01.
Created by Shane Parson, Copyright 1999.
