► In spring maize growing period of a sub-humid area, fallow field with wheat straw mulch had a fallow efficiency of 35%. ► Fallow field mulched with plastic sheets had a fallow efficiency of 46.1%. ► Largest water loss by evaporation occurred during the hottest part of summer. ► Maize plant canopy significantly reduced evaporation due to uptake of soil water and shading of the soil surface. A field experiment was conducted in a dry sub-humid area to study the effect of plastic sheet mulch and wheat straw mulch on water loss by evaporation (E) under fallow and cropped conditions and water use by transpiration (T) under cropped conditions. Results showed that during the entire spring maize (Zea mays L.) growing period with 305.1mm water of precipitation and irrigation from April 22 to August 28, fallow plots mulched with wheat straw conserved 106.9mm water in the 0–200cm soil layer with a fallow efficiency of 35% while those mulched with plastic sheets conserved 140.6mm water with a fallow efficiency of 46.1%. Although plastic film and wheat straw mulch significantly reduced water loss by E compared to non-mulch that had typically a fallow efficiency of 10–15%, water loss by E was still serious, with the largest water losses occurring during the hottest part of summer (July and August). During this period, it was difficult to reduce E, even when mulch was properly applied. In contrast, water losses due to E were much lower when maize plants were grown on the plots. In this case, maize plants continuously took up water from soil, leading to a reduction in the amount of soil water available for E. The large canopy shaded the soil surface and reduced water loss by evaporation. Only 20mm, or 6.3% water was estimated lost by evaporation for maize grown on plots covered with plastic mulch. We developed a regression equation between shoot dry matter and transpiration amounts from plastic sheet mulched plots to estimate water loss by E in non-mulched and wheat straw mulched plots. Results showed that non-mulched plots lost 30.2% and wheat straw mulched plots lost 24.5% of the water received during the maize-growing season to E.