# How to calculate runoff in Rainfall Analysis?

I have 30 years daily rainfall data. How can i calculate the mean monthly runoff, minimum monthly runoff and maximum monthly runoff with the available rainfall data using GIS?

I would like to use SCS curve number method for run off computation.

• Can you show us a sample of the daily data? Commented Dec 15, 2012 at 10:25
• I have ArcGIS version 9.3 with me. And i would like to use SCS curve number method for run off computation. But i am confused with what kind of statistics that i should adopt with the daily rainfall data for getting mean,minimum and maximum monthly runoff Commented Dec 15, 2012 at 12:27

This is a rather old question, however you might still like an aswer to it.

I don't think you can use a GIS-only approach for your problem. What you need is a precipitation / runoff model. If you want to use the SCS curve number method for runoff calculation, I could recommend the following model to you:

SWAT Model

For this model you will need at least a DEM, a stream network, precipitation data, land use classes and discharge measurements for calibration. It runs on various time scales and has been used in many different locations worldwide. It's freeware / open-source and has a decent support community online.

There are a lot of similar models that perform tasks like this. In case you're looking for something different, please edit your question in order to let us know.

Presume you have no snowmelt and no baseflow to worry about? Curve numbers only give respones to rainfall events.

OTBE there are models that do this. Often/mainly out of USDA. Suggest AGWA for openers. Try SWAT.

• Welcome to GIS @SE. It is generally accepted practice to add examples or links to your answers to help users who may see your answer later on. Commented Oct 20, 2014 at 0:36

GIS software is useful for tasks related to the geometrical aspects of runoff modeling, but not the hydraulic or hydrodynanic aspects. So you can use GIS software to create/process your DEM and to extract basin boundaries, flow lines, accumulation, etc. Even though these characteristics affect flow, they are essentially just geometric (static) properties of the terrain.

In contrast, runoff is essentially a dynamic process. Programs like SWAT derive numerical solutions to the hydraulic or hydrodynanic equations that govern the fluid flow. Of course, it solves those equations for the specific geometry that you supply.

There have been some attempts to link the two, but the best approach at this time is use different programs for the two steps.