Hydrology Expert

---
name: hydrology-expert
version: 1.0.0
description: Expert-level hydrology covering the water cycle, surface water hydrology, groundwater, watershed analysis, flood hydrology, water quality, and water resource management.
author: luo-kai
tags: [hydrology, water cycle, groundwater, floods, watersheds, water quality]
---

# Hydrology Expert

## Before Starting
1. Surface water or groundwater?
2. Flood, drought, or water quality focus?
3. Research or engineering application?

## Core Expertise Areas

### Water Cycle
Precipitation: rain, snow, hail — measured by rain gauges and radar.
Evapotranspiration: evaporation plus plant transpiration, largest water loss.
Infiltration: water entry into soil — controlled by soil type, moisture, vegetation.
Runoff: surface flow to streams — generated when precipitation exceeds infiltration.
Groundwater recharge: infiltrated water reaching saturated zone.

### Surface Water
Streamflow: discharge Q = A x V, measured at gauging stations.
Rating curve: relationship between stage and discharge at a gauging site.
Hydrograph: streamflow vs time response to precipitation event.
Unit hydrograph: 1 cm of effective rainfall over watershed in unit time.
Base flow: groundwater contribution to streamflow during dry periods.

### Groundwater
Aquifer types: unconfined (water table), confined (artesian), perched.
Darcy law: Q = -KA dh/dl, K is hydraulic conductivity.
Hydraulic head: elevation head plus pressure head.
Theis equation: drawdown around pumping well in confined aquifer.
Groundwater flow equation: combines Darcy law and continuity.

### Flood Hydrology
Return period: 100-year flood has 1% chance of exceedance in any year.
Frequency analysis: fit probability distribution to annual maximum flows.
Flood routing: tracking flood wave through channel or reservoir.
Rational method: Q = CiA for small watersheds, C is runoff coefficient.

## Best Practices
- Always perform quality control on streamflow records
- Use multiple methods for evapotranspiration estimation
- Account for land use change effects on hydrology
- Consider climate change projections in water resource planning

## Common Pitfalls
| Pitfall | Fix |
|---|---|
| Confusing return period with probability | 100-year flood does not mean once per century |
| Ignoring baseflow separation | Stormflow and baseflow behave differently |
| Applying Darcy law to turbulent flow | Check Reynolds number for flow regime |
| Assuming stationarity in frequency analysis | Climate change may shift flood frequency |

## Related Skills
- geology-expert
- meteorology-expert
- oceanography-expert