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Among traditional irrigation systems, pitcher irrigation is one of the most efficient. Water seeps out of a buried pitcher due to the pressure head gradient across the wall of the pitcher directly into the root zone of the irrigated crop. The pressure gradient results from positive pressure head inside pitcher and negative pressure head at the outer surface of the pitcher which is in contact with soil. Laboratory experiments were conducted to quantify the influence of evaporation, which controls the negative pressure condition in the soil, on the seepage rate of pitchers and to explore the auto-regulative capability of the pitcher irrigation system. Two groups of pitchers, group A with high hydraulic conductivity and group B with low hydraulic conductivity, were filled with water and placed in a closed chamber at varying temperature and humidity. The pan evaporation rate (Ep) inside chambers ranged from 1 mm/d when the temperature and relative humidity were kept around 20 degrees C and 97% to 16 mm/d at 45 degrees C and relative humidity of 40%. A positive and significant correlation was found between seepage rate of pitchers and Ep with an average R 2 of 0.97. The average seepage rates of all pitchers at Ep values of 1 and 16 mm/d were 125 mL/d to as high as 1020 mL/d, respectively. Differences in the seepage rates in pitchers from group A and group B were high at low Ep and low at high Ep values. At low Ep, seepage rate was more affected by water pressure head inside pitchers and the hydraulic conductivity whereas at high Ep values the negative pressure head at the outer surface of pitchers were more important. The average seepage rate of type A pitchers was 200% higher than that of type B at Ep value of 1 mm/d but for the high Ep value of 16 mm/d the corresponding increase in the seepage rate of type A compared to type B was only 4%. Thus, evaporation rate controls seepage from the pitcher and the system is auto-regulative. (c) 2006 Elsevier B.V. All rights reserved.