# Calculating terrain indices along streams

Most people would agree that streams have two sides, right? Funnily though, this fundamental truth has been up and until now completely overlooked by terrain analysts!

 Up and until now there was no GIS program around to calculate terrain indices for left and right stream sides separately. Distinguishing left and right is streams sides is fundamental because left and right streams sides and adjacent hillslopes usually have different characteristics including soil types, landuse and vegetation. It is those characteristics that can control the effectiveness of (near stream) riparian zones for releasing or retaining key compoundsÂ  of water quality such as nitrate, dissolved organic carbon or heavy metals. GIS-based terrain analysis has been often successfully employed to link environmental variables to remote sensed data and, thus, to upscale from the plot scale to the catchment scale. The SIDE (side-index division equations) algorithm has been specifically designed to enable usage of numerous terrain analysis techniques (such as calculating upslope area, upslope landuse percentages or topographic wetness indices) to describe riparian zones.The SIDE algorithm is implemented in SAGA GIS and can be downloaded with usage instruction on here. For more information about the methodology you can consult this WRR article (in press, link to come soon). Please cite this WRR article any time you use (or modify) the method… Read more below to find out if and how this method might be useful for you! or contact me if you seek for research cooperation!

## Calculating terrain indices along streams

Assume you want to know where and how many pesticides from agricultural land use enter a stream.

Imagine a case like the one below (loosely inspired from the title picture of this post):

Examining this map by eye is easy and there are some straightforward conclusions:

• Agricultural land-use influences only the right side of the stream
• Forests are located exclusively on the left side of the stream
• All cells labeled “A” receive water from 3 cells: 2 agricul. grid cells (right) and 1 forest grid cell (left)
• All cell labeled “B” receive water from 8 1/3 cells : 8 agricult. grid cells (right) and ~1/3 forest grid cell (left)
• All cell labeled “C” receive water from 2 1/3 cells: 3 agricult. grid cells (right) and ~1/3 forest grid cell (left)

Simple, huh? Repeating this for a map with several billions of grid cells is however a but more tedious. So, what about doing it automatically on a computer? Well, there are a few GIS programs that will allow to calculate “local contributions”, allowing you to see where and how much pollutant enters a stream. Just: Up and until recently there was no available GIS routine available to differentiate between stream sides! So, running a such a program would have given the following wrong answers:

• All cells labeled “A” receive water from 3 cells: 2 agricul. grid cells 1 forest grid cell (but who knows on which side?)
• All cell labeled “B” receive water from 8 1/3 cells : 8 agricult. grid cells and ~1/3 forest grid cell (but who knows on which side?)
• All cell labeled “C” receive water from 2 1/3 cells: 3 agricult. grid cells and ~1/3 forest grid cell (but who knows on which side?)

That seems similar, at first look. But what the difference interpretation of these results differs fundamentally different! Using the results obtained by-eye, one could for example deduce that:

• 14 km of soils along the stream are not polluted by agriculture! (Assuming a grid cell is 1km x 1km)
• Soils at the left stream side are (probably) rather dry, because most receive water from 1 upslope grid cell at most (which is half than what cells on the right side receive!)

What if we used a GIS routine that can’t distinguish left and right sides? I would guess that quite a number of users would simply assume that about 50% of water inflows are from left and 50% of water inflows come from right. That might sound reasonable but it really isn’t, and the conclusions would hence be:

• All soils along the stream are polluted
• Soils are (probably??) similarly wet on left and right stream sides and most receive water from ~1.5 grid cells

In short: Not distinguishing between left and right stream sides makes often no sense when using terrain analysis tools to asses stream landscapes. This is why the I designed and implemented anÂ  algorithm into the open-source software SAGA-GIS to, well, distinguish between left and right sides! If you want to learn more about the algorithm, please read the corresponding article (soon to be published). There is are also some basic usage instructions and a download link for the module in this post!