# Section 2: Hydrology
The land phase of the hydrologic cycle is based on the water balance equation:
$$
SW\_t = SW\_0 +\sum\_{i=1}^t (R\_{day} - Q\_{surf} - E\_a - w\_{seep} - Q\_{gw})
$$
where $$SW\_t$$ is the final soil water content (mm H2O), $$SW\_0$$ is the initial soil water content (mm H2O), *t* is the time (days), $$R\_{day}$$ is the amount of precipitation on day *i* (mm H2O), $$Q\_{surf}$$ is the amount of surface runoff on day *i* (mm H2O), $$E\_a$$ is the amount of evapotranspiration on day *i* (mm H2O), $$w\_{seep}$$ is the amount of percolation and bypass flow exiting the soil profile bottom on day *i* (mm H2O), and $$Q\_{gw}$$ is the amount of return flow on day *i* (mm H2O).
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