MRC-MDBC STRATEGIC LIAISON PROGRAM BASIN DEVELOPMENT PLANNING TRAINING MODULE 3 SCENARIO-BASED PLANNING for the MEKONG BASIN Napakuang, Lao PDR 8-11 December 2003 Hydrology Terms and Models (explained for non-hydrologists)

The Hydrological Cycle HYDROLOGY EXPLAINED The Hydrological Cycle

Rainfall (“precipitation”) and Runoff HYDROLOGY EXPLAINED Rainfall (“precipitation”) and Runoff Depth - millimetres (mm) - needs to refer to a period of time in which the rain occurred (eg the last 24 hours) or to the total rain in a particular rainfall event. Intensity - millimetres per hour (mm/hr) Duration - hours Runoff (the rainfall that doesn’t soak into the ground and turns into surface flows) - millimetres (as for rainfall), or flow, or volume

Probability Concepts HYDROLOGY EXPLAINED Probability Concepts Probability in hydrology refers to the chance of a specified hydrological event happening within a specified period of time. It is usually calculated as a percentage - that is, the chances in 100 of something happening. Can be though of as odds (like horse racing) 10% probability = odds of 9 to 1 against. Sometimes “return period” is used. A “1 in 100 year flood” is the size of flood that has a 1% chance of happening in any given year.

Some more hydrology terms HYDROLOGY EXPLAINED Some more hydrology terms Stage - river level (metres) Velocity - the speed of water past a point (metres per second - m/s) Discharge - what the rest of us call Flow - volume moving past a point in a given time - (cubic metres per second - “cumecs”) Hydrograph - a plot of flow or level against time. Flow can be instantaneous values or aggregated and averaged over time - eg “mean daily flow” - “average annual flow”. Time series - a set of data referenced by the time of each reading, sample or event.

Compound Probability Graphs HYDROLOGY EXPLAINED Compound Probability Graphs A commonly used compound graph for basin planning and for environmental flow purposes is the “flow duration curve”. From all the flow data available it is possible to aggregate it and compute the probability that any given flow occurs or is exceeded. The graph of these flows for exceedance probabilities from 0% to 100% forms the flow duration curve. Often the probability is computed on an annual basis and hence the term “annual exceedance probability” or AEP.

HYDROLOGY EXPLAINED

Variability Plotting

Hydrology Models HYDROLOGY EXPLAINED Hydrology Models Mathematical descriptions (relationships and equations) of the behaviour of a water system. The model is calibrated by testing how well the model replicates previously recorded events. By altering, for example, the input data to simulate changes in rainfall, a calibrated model will give a highly useful estimation of the future. For development scenarios, both data and relationships can be changed to simulate the behaviour of floodplain structures, dams, diversions etc and to estimate the outcomes.

1. Rainfall-runoff (converts one to the other) 2. River flow routing (linear models) - used for simulating the flow within river channels. Output is time series-data. Needs analysis. The analysis can include hydrographs, durations, frequency of occurrence etc Model is based on the sections of a river between selected nodes and works on ‘water balance’. Common Types of Model HYDROLOGY EXPLAINED Common Types of Model Water flow into section InflowsOutflows Water flow out of section pluslessequals

Common Types of Model (2) HYDROLOGY EXPLAINED Common Types of Model (2) Hydrodynamic - typically used for water flows that can flow out over the banks of a river. Used commonly in floodplain planning. Requires significant amounts of data about the terrain of the floodplain. Also works on water balance, but for two- dimensional cells rather than linear sections. (Three-dimensional models also exist for special tasks)

Models - Accuracy and Usefulness HYDROLOGY EXPLAINED Models - Accuracy and Usefulness Hydrology models have many assumptions in their internal workings and, of course, there is no such thing as perfect data. This may lead people to question their value. It is critical to remember that simulation models are used for comparisons of scenarios. It is a scientific fact that no matter the accuracy of the absolute outputs, the accuracy of the differences between runs is significantly higher. Simulation models are thus well accepted around the world and have been found to not just be useful for planning, but to be vital.

The Hydro-illogical Cycle HYDROLOGY EXPLAINED The Hydro-illogical Cycle FLOOD DROUGHT PANIC APATHY

THANK YOU Questions? I love my hydrological cycle!