## 1 Introduction  
Since a major incident involving contaminated milk in China in 2008, the scale of planting of high-quality forage grass has increased rapidly, in which alfalfa (_Melicago sativa_ L.) has been the major cultivar [1]. In 2014, nationally, forage grass was cultivated in more than half a million hectares, of which alfalfa accounted for more than 80%, with the corresponding increasing land use and the need for irrigation globally [2; 3; 4]. Alfafla consumes a lot of water during its growing season, usually around 300-2250 mm [5]. Apart from the limited natural rainfall [6], high-efficiency irrigation is necessary [7]. With the evolution of water-lifting devices [8], the center pivot sprinkler irrigation system (center pivot) has been a popular irrigation method worldwide [9]. Most of the new planting areas have been irrigated using the center pivot. Center pivots have the advantages of large-scale sprinkling operations, water and energy savings, and the integration of water and fertilizer [10; 11; 12]. Thus, they have played an important role in modern grass production [13; 14; 15; 16]. In 2010, the numbers of center pivots increased to 1100 [17]; there were 15,000 new installations in 2013, covering an irrigation area of nearly 400,000 \(\mathrm{h}\mathrm{m}^{2}\)[18]. According to our field survey in Ar Horqin Banner, Inner Mongolia, China, in a high-quality forage planting base the irrigated area of alfalfa (_Medicago sativa_ L.) and oats (_Avena sativa_ L.) has reached 53,000 hectares. There are more than 2000 center pivots. Recently, low pressure spray plates have replaced the previously used medium and high pressure sprinklers [19]. Two types of low-pressure sprinklers are commonly used with these center pivots: fixed spray plate sprinklers (FSPSs) and rotating spray plate sprinklers (RSPSs) [20; 21]. The deflection plate of an FSPS is fixed to the sprinkler body, and the streams are sprayed around the sprinkler by deflection by the spray plate. Here, the spray plate was engraved with a jet slot at a certain elevation to increase the radius of the wetted area. The spray plate of an RSPS was rotated under the drive of the streams to spray in the surrounding in a rotational manner. The basic performance parameters of the two types of sprinklers are shown in Table 1.  
As shown in Table 1, due to the structural differences of RSPSs and FSPSs, they exhibit different characteristics. According to the surveys, it was found that many entrepreneurs do not fully understand the differences in performance between RSPSs and FSPSs. Indeed, in sprinkler selection, more attention was paid to the difference in price between the two sprinklers. Although price is a factor in sprinkler selection for center pivots, the results of sprinkler selection can influence the sprinkling uniformity [22; 23; 24]. Sprinkling uniformity is important and affects the quality of sprinkling irrigation. Under conditions of low sprinkling uniformity, some irrigated areas will be applied excess water, and the crop yields and qualities will therefore not improve or even become degraded. However, the other area will suffer insufficient irrigation, and this also will cause the deceases of crop yields and qualities, thus leading to low efficiency for irrigation systems [25]. Center pivots play an important role in the integration of water and fertilizers, so its performance ultimately influences crop productivity and chemical characteristics in the rhizosphere [26]. Low sprinkling uniformity results in uneven growth of forage grass, reducing the commodity grade, and directly affecting the benefits of forage production [27; 28; 29]. Furthermore, the center pivot applies water over a fairly extensive area, so the problems of sprinkling uniformity in center pivot must influence the crop production in a wide range. Previous studies reported the water distribution characteristics of FSPSs [20; 30] and RSPSs [21; 31] separately under indoor conditions. Using a translational test device, [PERSON] et al.