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Deep drainage is defined as the volume of water flux passing below the depth at which plant roots extract water.

The importance of deep drainage is that it helps in understanding the amount of water being delivered to plants; it is also is a key indicator in water use efficiency and plays a significant role in the hydrological cycle. Without deep drainage excess salts accumulate in the root zone and a balance is needed and this derived from the Leaching requirement. Effects of deep drainage are evident through salinity, with at least 25,000 km² of Australia and 5% of current cultivated land is affected. Another environmental issue is the disposal of waste through the water flux such as dairy farms, industrial, and urban runoff.

Deep drainage methods can be divided up into:

• Physical,

Physical methods use the principles of soil physics to estimate deep drainage. The Physical direct methods are those that attempt to actually measure the volume of water passing below the root zone and the Physical indirect methods rely on the measurement or estimation of soil physical parameters, which along with soil physical principles, can be used to estimate the potential or actual deep drainage.

• Chemical,

Chemical methods utilise the presence of water-soluble substances, such as isotopic tracer, moving through the soil, as deep drainage occurs.

• Simulation Models, and
• Others.

• Drainage

• Allison G. B., Gee, G. W., and Tyler, S. W. (1994). Vadose-zone techniques for estimating groundwater recharge in arid and semiarid regions. Soil Science Society of America Journal 58, 6-14.
• Bond, W. J. (1998). "Soil Physical Methods for Estimating Recharge.," CSIRO Publishing, Melbourne.