## 3 Results and Discussion
However, the results in Figure 7 show that there are no significant differences in the values of \(T\) among the tested covers of the four prototypes. In fact, in actual greenhouses, the degradation of the optical properties of a duo-pitched-roof greenhouse is usually higher than that of a tunnel because the metallic structure of the duo-pitched roof is much higher than the tunnel structure. Metallic structures absorb solar radiation and release heat; this increases the cladding temperature and deteriorates its optical, chemical, and thermophysical properties faster. Moreover, the N surface of a greenhouse cladding usually receives indirect solar radiation with low intensity; consequently, the degradation of the N surfaces is expected to be lower than the other surfaces of cladding. In Figure 7, at the end of the exposure time (one year), the absolute loss in the transmittance (\(T_{\rm new}\)-\(T\)) was 28% for the N-S-direction duo-pitched roof cover and was 31% for the cover of the other prototypes (i.e., E-W duo-pitched roof type, N-S tunnel type, and E-W tunnel type). The relative loss in the cover transmittance was estimated from Equation (2) for the four covers tested and is illustrated in Figure 8. This figure indicates that for exposing a greenhouse (having any shape and oriented in any direction) to the arid climatic agents for one year, the loss in the cover transmittance is expected to be around 65-75% of its initial value (the new cover transmittance). The results in the previous figures show a drastic reduction in the transmittance of the plastic-film-covered greenhouses after one year of exposure, which means that such covers cannot serve without cleaning for more than one year in arid regions. This is because the transmitted radiation would not fulfill the growth requirements of most crops grown in the greenhouses. Thus, the cover should be cleaned immediately to improve its optical performance. This is because cleaning the LDPE film covering a greenhouse by washing the film using a water stream enhanced the cover transmittance by 7-11% at the same site of the present study's experiment [17]. Figure 9 illustrates the degradation rate of the cover transmittance with the exposure time (Equation (3)). The degradation rate processed rapidly in the first month of exposure; the rate of loss in the transmittance of the four covers tested reached 0.3-0.35% per day at the end of the first month and  
Figure 7: Average transmittance of the cover envelops (\(T\)), estimated for each prototype during the time of exposure to arid climatic agents. then drastically reduced to 0.07-0.1% per day at the end of the third month. After that, the degradation rate in the cover transmittance fluctuated between 0.01% and 0.1% per day for the four covers tested. The statistical analysis showed no significant differences in the degradation rate between the four cover surfaces (tunnel type) and between the six cover surfaces (duo-pitched roof type). Moreover, the degradation rates of the four prototypes were around 0.085%, and there were no differences between them, as shown in Table 1. This is because the tested greenhouse prototypes were in the same place and under the same climatic conditions, and the combined weather parameters have almost the same effect on the whole cover surfaces. On the other hand, it was difficult and expensive to use commercial greenhouses to perform the studies; therefore, small-size greenhouse prototypes were used to conduct the experiment. The results of the present study can be applied also for commercial greenhouses either duo-pitched roofs or tunnels oriented in the N-S or E-W directions. Figure 8: Relative losses in the cover transmittance estimated for the two greenhouse prototypes, oriented in the N–S and E–W directions and exposed for one year to the arid climatic agents. Figure 9: Time course of the degradation rate of the cover transmittance estimated for the two greenhouse prototypes oriented in the N–S and E–W directions and exposed to the arid climatic agents.