The pumping rate (¯owrate, f) depends basically on two factors: the pumping head, H, and the irradiance, I. Fig. 1 shows the dependence of the ¯owrate on both head and irradiance. Each line represents a dierent head. It is noted that the curves cross each other. This is attributed to the dierences in the pv module eciencies for dierent sets of data. The eect may also be partially caused by the dierences in the actual pump characteristics at dierent heads. The pump is less ecient at lower heads, simply because some energy is transferred to the pumped water in the form of kinetic energy. Ideally the pump should deliver water to a certain head with zero kinetic energy. In general the ¯owrate increases with irradiance, but not linearly. A second order parabolic equation ®ts the data very well. Table 1 shows the equations for the best ®t curves and the correlation coecients (r2).