Lysimeter Requirements(Jensen et al, 1990)

When lysimeter are used to measure actual evapotranspiration rates, it seems essential that they are either quite deep or fitted with a tensioning at the bottom, to allow a normal root growth;
They should contain an undisturbed, representative profile. In a disturbed profile, moisture transmission, moisture retention, and root distribution is likely to be different from that of the original profile and measurements may not be representative;
The vegetation inside and outside the lysimeter should be kept as similar as possible;
Diminishing the effect of the lysimeter rim over ET measurements by reducing the lysimeter wall thickness, the gap between inner and outer walls, and the height of the lysimeter rim relative to soil surface;
Reducing the oasis effect by providing sufficient distances of winward fetch of similar vegetation and soil moisture regimes.

Large and Mini Lysimeters

Large lysimeters are the standard instrument for measuring evapotranspiration (surface area >2 m²) (Slatyer and McIlroy, 1961). Recently many researchers have used ‘minilysimeters’ in field studies (Grimmond et al., 1992). They have the advantage that minilysimeters (1) permit the measurement of the evaporative flux from smaller areas; (2) create less disturbance to the environment during installation; (3) are cheaper to install than the large ones.

There are a big number of potential sources of error associated with using lysimeters, either associated with the mechanics and electronics of the lysimeter. In general, the effect of sources of error on the accuracy of evapotranspiration measurements is inversely related to the surface area of the lysimeter (Dugas and Bland, 1989).

Measurement and Estimation of ET_c

Southwest Florida Water Management District