Introduction

To understand the water regime of a specific area for water resource planning one of the first tasks is to understand the water balance of that area. Water balance is a budgeting exercise that assesses the proportion of the rainfall that becomes stream flow (or runoff), evapotranspiration, and drainage (or groundwater recharge).

• Read about constructing a water Balance
• Watch the video tutorial (Time: 21:00)- Part 1 & Part 2
• Download an excel file with an example of water balance 

Objectives of water balance demonstration    

1. To introduce the reader to a simple water balance model, namely, the Thorntwaite-Mather model, henceforth referred to as the T-M model (Thorntwaite et al, 1955;1957; Steenhuis et al, 1986);
2. To provide the reader with the tools to construct a water balance for her/his own region of interest, with the help of video tutorials and a sample excel spreadsheet that can be downloaded and modified.

This document is intended for general instructive purposes for an audience that has some basic knowledge of water resources and associated terminology. No advanced expertise should be needed to understand and use this tutorial.

There can be various models to construct water balance of an area; the model that is used here for demonstration is T-M Model which has an advantage of being one of the most simple models. It can be used to determine a general estimate of the water balance regime, for individual fields to small watersheds.

Applications and limitations : However, as in all scientific investigation, this tutorial should be used responsibly and with a full knowledge of the user's specific study area. This model and its variants have been used, for example, for irrigation scheduling of individual fields, water budgeting of small watersheds, generating actual evapotranspiration estimates for comparison with other methods - to name a few applications.

Being a lumped model, in the form described, the T-M model does not provide spatially distributed predictions, nor does it perform flow routing routines.

Authors:
Vishal K. Mehta (Arghyam Trust/Cornell University)
Dr. M. Todd Walter (Cornell University)
Dr. Stephen D. DeGloria (Cornell University)

References:
Allen, R. G., Pereira, L.S., Raes, D., Smith, M. (1998). "Crop evapotranspiration: Guidelines for computing crop water requirements" FAO Irrigation and drainage paper 56, Rome, Italy.
Thorntwaite, C. W., J. R. Mather (1955). "The water balance." Publ. Climatol. 8(1).
Thorntwaite, C. W., J. R. Mather (1957). "Instructions and tables for computing potential evapotranspiration and the water balance." Publ. Climatol. 10(3).
Steenhuis, T.S. And W.H. Van Der Molen. 1986. The thornthwaite-Mather procedure as a simple engineering method to predict recharge. J. Hydrol. 84:221-229.