# How to Analyze Hydrological Drought Using the Surface Water Supply Index (SWSI)

The Surface Water Supply Index (SWSI) is a valuable tool for analyzing hydrological drought. It integrates multiple hydrological components such as streamflow, reservoir storage, snowpack, and precipitation to provide a comprehensive measure of water supply conditions. Hereâ€™s a detailed guide on how to analyze hydrological drought using SWSI.

The Surface Water Supply Index (SWSI) is a valuable tool for analyzing hydrological drought. It integrates multiple hydrological components such as streamflow, reservoir storage, snowpack, and precipitation to provide a comprehensive measure of water supply conditions. Hereâ€™s a detailed guide on how to analyze hydrological drought using SWSI.

## Understanding the SWSI Formula

The SWSI formula is given by:

`SWSI = (100 * Pi - 50) / 12`

Where:
`Pi` is the cumulative probability (percentile) of the observed surface water supply compared to historical records.

## Steps to Analyze Hydrological Drought Using SWSI

### 1. Data Collection

Gather the following hydrological data:

• Streamflow: The flow rate of rivers and streams.
• Reservoir Storage: The volume of water stored in reservoirs.
• Snowpack: Snow Water Equivalent (SWE).
• Precipitation: Total rainfall over a specific period.

### 2. Historical Record Analysis

Compile historical records of the collected data. This historical dataset will be used to determine the cumulative probability `Pi` of the current observations.

### 3. Ranking and Cumulative Probability

Rank the historical records in ascending order and calculate the cumulative probability for each record. The cumulative probability `Pi` for a given value is determined by its rank in the historical dataset.

For example, if you have 30 years of historical data and the current year's value is ranked 5th, the cumulative probability is calculated as:

`Pi = Rank / (Total Number of Years + 1) = 5 / (30 + 1) = 0.161`

### 4. SWSI Calculation

Plug the cumulative probability `Pi` into the SWSI formula:

`SWSI = (100 * 0.161 - 50) / 12 = (16.1 - 50) / 12 = -2.825`

### 5. Interpreting the SWSI Values

Interpret the SWSI values using the following categories:

• Extremely Wet: SWSI > +3.0
• Very Wet: +2.0 < SWSI â‰¤ +3.0
• Moderately Wet: +1.5 < SWSI â‰¤ +2.0
• Slightly Wet: +1.0 < SWSI â‰¤ +1.5
• Near Normal: -1.0 â‰¤ SWSI â‰¤ +1.0
• Slightly Dry: -1.5 < SWSI < -1.0
• Moderately Dry: -2.0 < SWSI â‰¤ -1.5
• Severely Dry: -3.0 < SWSI â‰¤ -2.0
• Extremely Dry: SWSI â‰¤ -3.0

## Application in Hydrological Drought Analysis

The SWSI is used to monitor and manage water resources, providing insights into the severity and duration of drought conditions. Hereâ€™s how you can use the SWSI for hydrological drought analysis:

### 1. Real-Time Monitoring

Calculate the SWSI regularly (e.g., monthly) to monitor current water supply conditions. This helps in early detection of drought onset and allows for timely implementation of mitigation measures.

### 2. Trend Analysis

Analyze historical SWSI values to identify long-term trends and cycles in water availability. This can reveal patterns of recurring droughts and wet periods, assisting in water resource planning.

### 3. Decision-Making

Use SWSI values to inform water management decisions, such as:

• Water Allocation: Adjust allocations for agricultural, industrial, and municipal uses based on current and projected water availability.
• Drought Mitigation: Implement water conservation measures and restrictions during periods of low SWSI values.
• Reservoir Management: Optimize reservoir operations to balance water supply needs with environmental requirements.

## Case Study Implementation

### Objective

Assess the hydrological drought conditions in a river basin over the past decade using SWSI.

### Steps

1. Data Collection: Gather data on streamflow, reservoir storage, snowpack, and precipitation for the past 10 years.
2. Historical Record Analysis: Compile historical records for the past 30 years.
3. Cumulative Probability Calculation: Rank the historical records and calculate the cumulative probability `Pi` for each year.
4. SWSI Calculation: Use the SWSI formula to calculate the index for each year.
5. Trend Analysis: Plot the SWSI values over the decade to identify trends.
6. Decision-Making: Develop a drought management plan based on the findings.

### Example

Letâ€™s assume we have the following data for the current year:

• Streamflow: 1500 cfs
• Reservoir Storage: 600,000 acre-feet
• Snowpack: 80% of normal
• Precipitation: 75% of normal

And the historical ranking and cumulative probability `Pi` is 0.161 as calculated earlier.

`SWSI = (100 * 0.161 - 50) / 12 = -2.825`

This value indicates moderately dry conditions, prompting the implementation of water conservation measures.

Effectively communicate the results of the SWSI analysis to stakeholders:

• Executive Summary: Provide an overview of the findings and implications.
• Detailed Analysis: Include data, calculations, and trend analysis.
• Recommendations: Offer actionable recommendations for drought mitigation and water management.

The Surface Water Supply Index (SWSI) is a crucial tool for analyzing hydrological drought. By integrating multiple hydrological components and using historical data to determine cumulative probabilities, the SWSI provides a comprehensive measure of water supply conditions. Regular monitoring, trend analysis, and informed decision-making based on SWSI values can help mitigate the impacts of drought and ensure sustainable water resource management.

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