Groundwater and baseflow covering aquifers and their properties, gaining and losing streams, governing equations for groundwater flow, baseflow separation, baseflow models, parameter estimation, exponential decay and linear reservoir model.

Base flow is the rate of flow that a given river basin can sustain in the absence of precipitation and artificial storage works. Such flows are important in connection with water supply and water quality in rivers during drought periods, and general basin and agricultural drainage. But even storm runoff is largely supplied into the streams by subsurface transport. Thus, subsurface drainage from the aquifers along the banks of the streams is one of the key elements in catchment hydrology, not only under drought conditions but also in response to precipitation. Herein the subsurface outflow is first considered locally at the point where it enters the stream, by analysis of the groundwater flow process in the riparian unconfined aquifer. Thus, the different available formulations are reviewed; these comprise general unconfined flow, free-surface flow, and hydraulic groundwater theory and its linear approximation, including flow in sloping aquifers. In the last section, the base flow is parameterized at the basin scale, by integration of the local outflows along all the streams in the basin. The base flow trend of a basin provides an indication of its groundwater storage changes.

Groundwater is of vital importance for agricultural irrigation. In almost all countries, a significant portion of irrigation water is derived from groundwater using wells. In developing countries where farm holdings are small, one well may suffice but a number of wells are used in large farms. This chapter discusses rudimentary aspects of groundwater and wells from the perspective of irrigation. Although the material covered in the chapter is based on gross simplifications, it has been found to be useful nonetheless for irrigation purposes.