## 1 Introduction  
Application efficiency (Ea) at the plot level is strongly related to the uniformity of water distribution and the irrigation method used [1]. In sprinkler irrigation, the following are the most important factors determining Ea [2, 3, 4]: (a) distribution uniformity on the irrigated area and (b) drift losses due to drift of droplets, together with droplet shape and size, influenced by environmental conditions (radiation, temperature, relative humidity and wind speed). Factors (a) and (b) largely depend on the characteristics of the sprinklers used (spray, rotator, nozzle diameter, spacing, distribution pattern), their position relative to the crop canopy (below or above the crop) and working pressure, as well as on weather conditions at the moment of irrigation (especially wind speed); (c) evaporation, either from the soil or the plant; (d) plant leaf architecture and (e) soil characteristics (infiltration, amount and composition of crop residues, slope gradient). The equation proposed by [2] is one of the most widely used for estimating \(\mathrm{E}\mathrm{a}\) in solid-set irrigation systems, but it has been less widely used in center pivot systems [5]. The equation considers the following components: (a) losses related to damage or leaks in the system pipelines; (b) distribution uniformity for a percentage of well-irrigated area (DUpa) and (c) evaporation and drift losses of sprinkled droplets. The interception of irrigated water is influenced by crop type regarding leaf inclination (erectophile or planophile), which determines the mode in which water reaches the soil [6; 7]. Other influencing factors include the water distribution model, sprinkler spacing and height relative to the canopy [8]. In turn, water applied can reach the soil directly, flow through the stem or store on the crop canopy temporarily and then evaporate [4]. These authors developed different equations to quantify irrigation water partitioning during the flow to the soil; the equations were proposed for high-pressure impact sprinklers and rotating low-pressure sprinklers based on the knowledge of the applied depth and plant spacing along the crop line. However, losses due to canopy interception are not incorporated into a general sprinkler irrigation efficiency equation. In the Argentine Pampas plains, direct sowing is a widely used production system [9; 10]. This management practice contributes residues and increases both infiltration and soil hydraulic conductivity [11] while reducing evaporation from the soil surface [12]. The application of supplemental irrigation with a center pivot helps stabilize the yields of crops, such as wheat, maize, and soybean, which also contribute important amounts of residues [13]. Tissue density has an important role in water retention and transport [14], yet there is no consensus in the literature about the water storage process, known as retention in residues [15; 16] or interception [17]. In the latter work, wheat residue was found to retain a significantly higher amount of rainwater (5 to 7 mm) than corn and soybean residues (2.5 and 3.5 mm, respectively). The maximum water retention in plant residue decreases with the increase in decomposition, an effect strongly associated with lignin concentration in residues [16], and water retention is highly correlated with the generation of pores during decomposition rather than the tissue composition of harvest residues (lignin, cellulose and hemicellulose) [11; 18]. Despite the influence of harvest residues on water retention, this variable has not been incorporated into an equation to calculate sprinkler irrigation efficiency. Correctly quantifying total irrigation losses in a center pivot system is important for determining the gross depth and improving irrigation management and efficiency, especially in areas where water is a scarce and/or expensive resource. Losses due to evaporation, wind drift and lack of uniformity in a center pivot system can be estimated using the equation proposed by [2].