Components of the water balance in soil with sugarcane crops

The objective of this study was to analyze the components of the water balance in an Ultisol, located in the municipality of Jaboticabal, SP, Brazil (21°20′20″S, 48°18′35″W), that was cultivated with sugarcane. The monitoring was performed during the agricultural cycle of the first ratoon between 11/16/2006 and 7/9/2007. Three treatments were established in four blocks with doses of ammonium sulfate, as follows: Treatment 1 (T1), without fertilizer; Treatment 2 (T2), 100 kg ha−1 of nitrogen (N) and 114 kg ha−1 of sulfur (S); and Treatment 3 (T3), 150 kg ha−1 of N and 172 kg ha−1 of S. Rainy precipitation (P) in the area was measured with a rain gauge. The soil water storage (H) and the soil water storage variations (ΔH) were determined by the gravimetric method, and the internal drainage (D)/capillary rise (CR) at a depth of 0.9 m was quantified by the water flux density using the Darcy–Buckingham equation. The actual evapotranspiration (ETa) was calculated as follows: ETa = P − D + CR ± ΔH. During the study period, the amount of rainfall was 1406 mm, 121 mm greater than the historic average for the region (1285 mm), with a notable peak in the month of January of 402 mm (historic average: 251 mm). The internal drainage was 300 mm under T1, 352 mm under T2, and 199 mm under T3, and this was relevant during times with elevated P, when the actual H was greater than the field capacity H. The actual evapotranspiration (T1: −897.7 mm, T2: −847.5 mm, and T3: −970.8 mm) and the water use efficiency (T1: −131.3 kg mm−1, T2: −146.6 kg mm−1, and T3: −127.5 kg mm−1) did not differ among the treatments. The dispersion of D was greater than the other components of the water balance, especially during the period of elevated P, with the errors of this process propagated in the estimation of ETa. Despite of this propagated standard deviation of ETa, it accounted less than 15% of the total ETa, showing that the method may be conveniently used in field studies with sugarcane crops.

► We study the water balance components in an Ultisol cultivated with sugarcane. ► The dispersion of internal drainage was greater than the others components. ► The errors were propagated in the actual evapotranspiration estimation. ► Water use efficiency betewen fertilized treatments were not different.