## 4 Conclusions  
In arid regions, plastic films are used extensively to cover greenhouses because of their beneficial effect on the sustainability, quality, and productivity of crops. However, greenhouse growers face serious problems: fast degradation of the optical performance of the covers (drastic reduction in the cover transmittance) and the need to replace the cover immediately, which in turn increases the production cost. This study described the degradation characteristics of the optical properties of different surfaces of the covers of four greenhouse prototypes (two duo-pitched roof-shaped and two tunnel-shaped), oriented in the N-S and E-W directions, and exposed for one year to arid climatic agents. The main conclusions that could be drawn from the present study are as follows:  
1. In some cases, the N and W cover surfaces showed a relatively lower degradation rate (i.e., rate of reduction in transmittance \(T_{\text{s}}\)) than the other surfaces. However, in general, the climatic agent (combined effect of solar radiation intensity, dust accumulation, dusty wind, etc.) has nearly the same effect on the degradation rate of the cover surfaces. 2. The shape or orientation of the prototypes had no significant effect on the degradation rate of the whole cover transmittance (\(T\)) to solar radiation. This is because the tested prototypes are small in size, and the surfaces are exposed nearly to the same weather conditions. Therefore, greenhouses of large size are recommended for such studies. 3. For the four covers tested, exposing them to arid climatic agents for one year reduced the transmittance of the cover (\(T\)) to global solar radiation by about 27-31% and increased the reflectance (\(R\)) by about 15-20% compared to the new film. Moreover, one-year exposure under a harsh arid environment is enough time to investigate the degradation behavior of the cover films. 4. Nonlinear regression correlation was presented (as a function of cumulated solar irradiance) to predict the cover transmittance to global solar irradiance at any time during the serving life of the film; the correlation is valid for any greenhouse, having any shape or orientation, under arid climatic conditions. Figure 11: Degradation of the cover transmittance (\(T\%\)), for the four prototypes tested, affected by the cumulative incident solar radiation flux (MJ m\({}^{-2}\)) during the exposure time, from 1 October 2019, to 1 October, 2020. * In order to clearly determine the lifetime of each cover surface individually, further research should be conducted on large-size greenhouses to predict the degradation characteristics of each surface, as well as the optical constants (i.e., refractive index and extinction coefficient) under arid climatic agents. Conceptualization and methodology, A.M.A.-G., P.P. and I.A.-H.; validation, A.A.A. and A.I. ; formal analysis, P.P. and I.A.-H.; investigation, A.M.A.-G.; resources, Shady; data creation, M.S. and A.I. ; writing--original draft preparation, I.A.-H. and A.I. ; writing--review and editing, A.M.A.-G. and P.P. ; visualization, M.S. and A.I. ; supervision, I.A.-H.; project administration, A.A.A. All authors have read and agreed to the published version of the manuscript. The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the research group No. RG-1435-074. The authors declare no  
* _[PERSON] and [PERSON] (2005)_ _[PERSON]; [PERSON] Testing of the condensation characteristics and light transmissions of different plastic film covering materials._ Polym. Test.__**2005**, _24_, 284-289. [CrossRef]
* [PERSON] (1978) [PERSON] Effect of dust on flat plate collectors. In _Sun: Mankind's Future Source of Energy_; Elsevier: New York, NY, USA, 1978; Volume 2. * [PERSON] (1994) [PERSON] The effect of dust accumulation on the performance of evacuated tube collectors. _Sol. Energy_**1994**, _53_, 105-115.