Large lift irrigation plants (LIPs) are a recent development in parts of Gujarat. These are commonly sited on old tanks, some on intermittent streams and canals. Water is lifted to a high point of command through a riser and then led by gravity through pipes to various distribution chambers. A need had arisen to examine their performance. Accordingly, a continuos, deterministic mathematical model of a typical LIP has been made. The system is viewed as a negative feedback, automatic (on/off) control. Curve number method is used to compute runoff and moisture balance method, for effective rainfall. Simulations using actual (historical) daily rainfall were carried out to determine irrigation needed and possible. Long term value of latter is taken as a more realistic index of system capability than that originally projected. Actual performance is compared with it. Possible additional uses of simulation in design and operations are highlighted.