Oil palm is one of the most rapidly growing tree crops in the tropics. It is long-lasting and high yielding, serving as an input for a number of profitable industries. The rapid expansion of oil palm has triggered environmental change. Historically, the focus has been on the impact of biodiversity loss. However, the water requirements of oil palm plantations, which traditionally depended on rainfall only, are also changing, partly because environmental concerns are directing oil palm expansion and cultivation into marginal areas. According to some estimates, these lands with cultivable marginal soils— having an acid pyrite layer in the soil profile— comprise about 7.5 million ha in Indonesia. Here, we employed the Agricultural Production Systems sIMulator (APSIM) to simulate the growth of oil palm on marginal lands within Indonesia over an eight year period. APSIM-Oil palm was used to estimate the irrigation water requirement at different stages of plant growth for actual weather and soil conditions. Traditionally, water footprint accounting of oil palm plantations at the field level considers one uniform value of evapotranspiration. Our analysis shows that considering a single value for the entire period of oil palm growth underestimates the water requirement at the field scale. Annual irrigation needs were found to range from 2543 mm to 3865 mm for the plantation ages examined (0-8 years). We approximate that 8800 m ³ of blue water and 6200 m ³ of green water is required per ton of fresh fruit bunch produced from the study plantation occupying marginal lands, where water requirements were largely governed by maintenance of a high water table. Similarly high volumes are likely to be required where oil palm is cultivated on pyritic soils. Thus, the irrigation water requirement can no longer be neglected as oil palm plantations continue to expand onto marginal soils.