Two field experiments were carried out at Wadi El-Natrun, El-Behera Governorate, during 2008 and 2009 seasons to study the optimum tomato yield and net return obtained by irrigation water amounts and nitrogen rates under drip irrigation system. Split plot design was used with four replicates. The main plots were assigned by four irrigation water amounts (100 % , 90 % , 80 % and 70 %) of evapotranspiration ( ETc ). The sub-plots were randomly assigned by four nitrogen rates (0 (N0), 150 (N2), 225 (N3) and 300(N4) kg N fed.-1) as ammonium nitrate.
The other recommended agriculture practices were done .
Four polynomial quadratic equations were established to show the following results:
1. The maximum and optimum N rates ( Xmax and Xopt ) were increased as irrigation water amounts decreased from 100 % of ETc to 90 % of ETc and decreased as irrigation water amounts decreased from 90 % to 80 and 70 % of ETc in the two seasons.
2. The maximum and optimum tomato yields ( Ymax and Yopt ) were decreased as irrigation water amounts decreased in the two seasons.
3. The highest maximum yield (44.359 ton fed.-1), the optimum yield (44.260), the highest return value of N fertilizer ( 10933.5 L.E fed.-1 ) and the highest net return of N fertilizer ( 9072.0 LE fed.-1) were obtained as irrigation water amount 100 % of ETc used in the two seasons.
4. The efficiencies of N rates ( eX) were decreased as N rates increased from N0 to, N1, N2, N3 and N4 respectively with different irrigation water amounts .
5. The average of efficiency (), the relative efficiency ( EX ), the efficiency of nitrogen fertilizer at optimum rate (eXopt ) and the efficiency of soil nitrogen (eXs ) were decreased as irrigation water amounts decreased .
6. The soil nitrogen content during plant growth (Xs) was decreased as irrigation water amounts decreased .
7. The contribution of soil N was decreased as irrigation water amounts decreased
in the two seasons.
8. The contribution of N fertilizer was increased as N levels increased in the two seasons .