# Sediment Delivery Ratio - very high values

Hello,

I ran the Sediment Delivery Ratio model with InVEST 3.2.0

for a study area in the Ethiopian highlands obtaining very high values. For

example the usle ranges between 0 and almost 500000 with a mean of 3500. For

comparison I calculated the usle with GRASS GIS using the same data. In this

case the results are lower, between 0.6 and 18000 with a mean of 240. With

further investigation I realised that maps of length slope calculated by InVEST

and GRASS (using r.watershed module, https://grass.osgeo.org/grass70/manuals/r.watershed.html)

are very different. In GRASS GIS the minimum slope length is 0.03, maximum = 24

and mean = 0.4, while in Invest minimum

= 0.05, maximum = 1174 and mean = 8.5.

I am wondering if this is expected behaviour of InVest ? In

this case how this can be explained? Or

is there something that I am missing?

Thanks in advance for your help.

Stefania

## Comments

thanks for your reply.

I'm working with a DEM of 92 m.

Unfortunately I couldn't find the work used by GRASS GIS to calculate the LS factor (Weltz et al., 1987).

Thanks,

Stefania

Ref: Weltz M. A., K. G. Renard, J. R. Simanton (1987).

Revised Universal Soil,Loss Equation for Western Rangelands

U.S.A./Mexico Symposium of.Strategies for Classification and Management of Native Vegetation for

Food Production In Arid Zones

Hello,

sorry for the late reply.

I have just started working with the 30m SRTM DEM. When I started running the

models it was not available.

My K-factor is far higher than yours. It ranges approximately from 0.3 to 0.4.

How did you calculate it?

I used the equations in Zhang et al.,

2008.

Cheers,

Stefania

REF: Zhang et al., 2008. Soil erodibility and its estimation for agricultural

soils in China. Journal of Arid Environments, 72: 1002-1011.

Catena,118, 220-225.Catena.thanks for your reply and for all the advices!

Sorry I forgot to indicate the equation that I used of Zhang et al. (2008). I didn't use the Eq. 6 but I used the Eq. 5, the one with silt, clay, sand and carbon %. According to this equation my values should be in SI units.

I'll read your references soon, thanks.

Stefania