As a result of low and erratic precipitation in drylands, ephemeral or intermittent streams are the norm rather than perennial streams. It is not uncommon for perennial streams to originate in higher elevation zones, or from springs or seeps at the toe of slopes in mountainous areas, and then become intermittent downstream from the source area. Transmission losses of water (i.e., water that infiltrates into the channel bottom) can, however, result in a mounding of shallow groundwater below and near stream channels and in adjacent floodplains.

Dense vegetative communities in these riparian zones, often dominated by phreatophyte vegetation (water-loving plants, often deep rooted trees), are indicators of such conditions. Saltcedar (Tamarix spp.), Mesquite (Prosopis spp.), and cottonwood (Populus spp.) are examples of phreatophytes which have been shown to transpire 1,000 mm to more than 2,000 mm of water per year from shallow water tables in the southwestern United States (Horton and Campbell 1974; Van Hylckama 1970). During periods of prolonged rain, or snowmelt runoff from high elevation zones, the water table can rise to the surface channel, only to retreat below stream channel bottoms during dry periods as a result of excessive evapotranspiration by phreatophytes. As a result, perennial streams in downstream valleys are rare as rainfall or snowmelt cannot be relied upon to sustain streamflow throughout the year.

...However, the total renewable water supply from drylands is estimated to constitute around only 8% of the global renewable water supply (about 3,200 km3 per year, Vorosmarty 2004), and only about 88% of this is accessible for human use. Thus, almost one-third of the global population depends on only 8% of the global renewable water resources, which makes per capita availability in drylands of 1300 m3 per year. It is substantially less than the average global availability and even lower than the 2000 m3 regarded as a minimum.