Indicators of Hydrologic Alteration (IHA)

Indicators of Hydrologic Alteration (IHA) is a software program that provides useful information for those trying to understand the hydrologic impacts of human activities or trying to develop environmental flow recommendations for water managers. Nearly 2,000 water resource managers, hydrologists, ecologists, researchers and policy makers from around the world have used this program to assess how rivers, lakes and groundwater basins have been affected by human activities over time or to evaluate future water management scenarios.

This program was developed by scientists at The Nature Conservancy to facilitate hydrologic analysis in an ecologically-meaningful manner. The software program assesses 67 ecologically-relevant statistics derived from daily hydrologic data. For instance, the IHA software can calculate the timing and maximum flow of each year's largest flood or lowest flows, then calculates the mean and variance of these values over some period of time. Comparative analysis can then help statistically describe how these patterns have changed for a particular river or lake, due to abrupt impacts such as dam construction or more gradual trends associated with land- and water-use changes.

For more information on IHA, including documentation, publications and to download the stand-alone version of the software, go to  

Much of the functionality of the stand-alone version of IHA has been incorporated into WEAP.

All users are encouraged to read the IHA User Manual, which gives a very good background on the theory behind IHA, explains key concepts and terms such as Environmental Flow Components (EFC) and the Range of Variability Approach (RVA), and details the algorithms used.  These topics are beyond the scope of the WEAP User Guide, but are common between the stand-alone IHA software and the implementation within WEAP, and are crucial for understanding how to use them.


To configure your IHA analysis, from the menu, go to Advanced, Indicators of Hydrologic Alteration (IHA).  From the Results View, click the "IHA Settings..." button at the top (when you are viewing IHA results). When making changes to settings from the Results View, WEAP will recalculate immediately.  On the IHA Setup screen you can choose analysis type and years, types of statistics and RVA boundaries, and definition of environmental flow components.

Analysis Years, Analysis Types

An IHA analysis can either look at streamflow statistics for a single period, or can compare statistics for two different periods or two scenarios.  The streamflow to analyze can either come from historic records, which would read from a CSV file into a gauge object, or from modeled results at any node in the river.

The two-period analysis typically splits the record into a period before substantial alteration of the historic streamflow ("pre-impact"), which could be caused by reservoir construction or re-operation, withdrawals from the river, or land use or climate changes that would influence runoff to the river, and a period after the alternation had begun ("post-impact").  In this way, historical changes to streamflow can be quantified and characterized.  If a historical streamflow record is not available, you can use WEAP's catchment hydrology to reconstruct the natural streamflow that existed before alteration, for comparison to the post-impact flows.  The pre-impact time period can be before the Base Year if the CSV files read by the gauges include data for this period.  For river nodes with a gauge immediately downstream, the pre-impact streamflow will come from the gauge's CSV file; otherwise, the pre-impact streamflow will be the modeled streamflow results at that river node (this works only if the pre-impact period is NOT before the Base Year).

In a two-scenario analysis, you can construct scenarios of change and use IHA to analyze their impact on historic or current flows.  Choose the scenario that will be the reference scenario (typically, this will represent the pre-impact state, or a baseline that you want to compare against).  Flows from the reference scenario will be used to calculate the Environmental Flow Components thresholds for the RVA analysis.  

IHA requires daily streamflow for a full analysis -- therefore, a WEAP model with a daily timestep is recommended.  (If the model has a monthly timestep, there will only be a partial IHA analysis, with monthly mean flows only.)

On the Analysis Years tab of the IHA Setup screen, choose Analysis Type (Single Period, Two Period, Multiple Scenario Comparison) and Years .  For years, you can set the overall years to use (Default Years), with the option to override the defaults for any specific river nodes (Node-Specific Years).  For example, if several reservoirs were each built in different years, you might set the boundary between pre-impact and post-impact differently for each reservoir to correspond to each reservoir's years of construction.  Note: these years are water years, which are different from calendar years if your WEAP water year does not begin in January.  If the node-specific setting is blank, it will default to the setting for the node upstream (on the same river).  If the first node on a river is blank, it will default to the area-wide default setting.  


As described in the IHA Manual, "IHA parameters can be calculated using parametric (mean/standard deviation) or nonparametric (percentile) statistics. For most situations non-parametric statistics are a better choice, because of the skewed (non-normal) nature of many hydrologic datasets (a key assumption of parametric statistics is that the data are normally distributed). But for certain situations, such as flood frequency or average monthly flow volumes, parametric statistics may be preferable."

The Range of Variability Approach (RVA) compares the variation in the IHA parameters from the pre-impact period to the variation in the post-impact period (or reference vs. alternative scenarios) to determine the extent of the changes.  Each IHA parameter is analyzed to determine the frequency with which it falls into one of three RVA categories (Low, Middle, High), as defined by the RVA Category Boundaries.  RVA requires at least two years of pre-impact data.

The change in frequency from pre-impact to post-impact of each IHA parameter in each of the three RVA categories is reported as the Hydrologic Alteration (HA) for that parameter.  The factors are calculated by the equation: (observed frequency expected frequency) / expected frequency, where "expected frequency" is the frequency expected in the post-impact period if it followed the same pattern as the pre-impact period, and "observed frequency" is the frequency actually observed in the post-impact period.  A Hydrologic Alteration factor greater than zero indicates increased frequency (from pre- to post-), whereas HA less than zero corresponds to decreased frequency.

On the Statistics tab, choose the Type of Statistics you want.  You can also set the RVA Category Boundaries and High Flow Pulse and Low Flow Pulse Thresholds, both defaults and, optionally, specific values for individual river nodes.  Note: RVA is not calculated for single-period analyses.

Environmental Flow Components

IHA defines five major components of flow that are ecologically important: extreme low flows, low flows, high flow pulses, small floods, and large floods.  Environmental Flow Components are only available for models with a daily timestep.  On the Environmental Flow Components tab, enter values to define the thresholds for these five components, both the defaults and any node-specific overrides.  The thresholds can be defined as % of mean daily flow, a specific CMS or CFS rate, or, in the case of small and large floods, as a N year return event.  For example, if a large flood is one that occurs on average every ten years, enter 10 year return event for the Large Floods category.  For single-period analyses, the EFC thresholds are computed based on the entire period; for two-period or two-scenario analyses, the EFC thresholds are computed based on the pre-impact period or reference scenario, and then applied to the post-impact period or alternative scenarios.  The categories Extreme Low Flows, Small Flood and Large Floods are optional.

Menu option: Advanced, Indicators of Hydrologic Alteration (IHA)