Water use assessment in muskmelon by the Penman-Monteith "one step" approach

The use of the “one-step” approach of Penman–Monteith (P–M) to assess crop water use has its limit in the lack of user friendly methodologies for a daily assessment of crop canopy resistance, rc. The model proposed by Monteith [Monteith, J.L., 1965. Evaporation and environment. In: Fogg, G.E. (Ed.), The State and Movement of Water in Living Organism. Soc. Exp. Biol. Symp. 19, 205–234] to estimate rc (rc = 100/LAI), although it is simple, requires the knowledge of LAI value during growing cycle. This work aims to propose an easy-to-use methodology for irrigation scheduling, to assess the values LAI gets during the growing cycle and thus for canopy resistance assessment. In muskmelon crop, grown with and without plastic film mulching, canopy resistance was measured by P–M formula inversion, clarifying the unknown resistivity term, being crop evapotranspiration known by lysimeter-based measurements. Measured canopy resistance was compared with the one estimated by Monteith model (1965), both in its original version and in the form we simplified in terms of LAI value during crop cycle. Thus, we compared the evapotranspiration assessed by the “one-step” use of P–M equation, with those estimated by the classical “two-step” approach, using crop coefficients, and with that directly measured by lysimeters. In particular in the mulched crop the “one-step” approach of P–M overestimates water use only by 4%, while the “two steps” approach underestimates water use by 17%. Despite both the methodologies proposed for LAI calculation and the Monteith model to assess canopy resistance extremely simplify more complex processes, they were able to give a good accuracy to assess muskmelon water use by the P–M “one-step” equation. Comparing the “one-step” and the more used “two-step” approaches it came out that, despite the latter is better correlated to the data measured by a lysimeter, it does not achieve a more accurate assessment compared to the “one-step” approach; in particular, in the mulched crop, the “two-step” approach significantly underestimates water use, while its estimation is reliable with the “one-step” approach.