Topographic Wetness Index Calculator
Calculate topographic wetness index with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Formula
TWI = ln(a / tan(beta))
Where a is the specific upslope contributing area per unit contour length (upstream area divided by cell size, in meters), and beta is the local slope angle in degrees. The natural logarithm of the ratio gives the TWI value, with higher values indicating greater moisture accumulation potential. Modified TWI includes soil transmissivity: TWI_mod = ln(a / (T * tan(beta))).
Worked Examples
Example 1: Hillslope Position Assessment
Problem: A point on a hillslope has an upstream contributing area of 10,000 sq m, is on a 10-degree slope, and the DEM cell size is 30 m. Calculate the TWI and assess saturation potential.
Solution: Specific catchment area (a) = 10,000 / 30 = 333.33 m\nSlope in radians = 10 x pi/180 = 0.1745 rad\ntan(10 degrees) = 0.1763\nTWI = ln(333.33 / 0.1763) = ln(1890.5) = 7.544\nSaturation class: Moderate (TWI between 6 and 9)
Result: TWI = 7.544 | Moderate saturation potential | Mid-slope position
Example 2: Valley Bottom vs Ridgetop Comparison
Problem: Compare TWI for a valley bottom (upstream area 50,000 sq m, slope 3 degrees) versus a ridgetop (upstream area 500 sq m, slope 20 degrees). Cell size 30 m.
Solution: Valley bottom:\na = 50,000/30 = 1666.67 m, tan(3) = 0.0524\nTWI = ln(1666.67/0.0524) = ln(31,806) = 10.37 (High saturation)\n\nRidgetop:\na = 500/30 = 16.67 m, tan(20) = 0.3640\nTWI = ln(16.67/0.3640) = ln(45.8) = 3.82 (Low saturation)
Result: Valley: TWI = 10.37 (High) | Ridge: TWI = 3.82 (Low) | Difference = 6.55
Frequently Asked Questions
What is the Topographic Wetness Index and what does it measure?
The Topographic Wetness Index (TWI) is a steady-state hydrological index that quantifies the tendency of water to accumulate at any point in a landscape based on topography alone. Developed by Beven and Kirkby in 1979, TWI combines the upslope contributing area (how much water flows toward a point) with the local slope (how quickly water drains away). Higher TWI values indicate locations where water is likely to accumulate, such as valley bottoms and flat areas with large upslope contributing areas. Lower TWI values correspond to well-drained locations like hilltops and steep slopes. TWI is widely used in hydrology, ecology, soil science, and geomorphology as a proxy for soil moisture patterns.
What applications use the Topographic Wetness Index in environmental science?
TWI has numerous applications across environmental science disciplines. In hydrology, it helps predict zones of saturation and variable source areas that contribute to storm runoff. In soil science, TWI correlates strongly with soil moisture, organic matter content, soil depth, and nutrient availability patterns across landscapes. Ecologists use TWI to predict plant species distributions and vegetation patterns because many species are sensitive to soil moisture gradients. In precision agriculture, TWI maps help optimize irrigation scheduling and identify areas prone to waterlogging. Geomorphologists use TWI to identify areas susceptible to landslides and mass movements. Environmental engineers employ TWI in wetland delineation, non-point source pollution modeling, and watershed management planning.
How is the heat index calculated?
The heat index combines air temperature and relative humidity to determine perceived temperature. The NWS uses a regression equation with nine terms. At 90F with 60% humidity, the heat index is about 100F. Heat index values above 105F indicate danger. Direct sunlight can add up to 15F to the heat index value.
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You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.
What formula does Topographic Wetness Index Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.
Is my data stored or sent to a server?
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