## 1 Introduction  
The sun drying of paddy rice is a common practice in the Philippines and many other countries in the tropics and subtropics. Immediately after harvesting/threshing, paddy rice is spread over mats positioned alongside roads or on other paved ground when weather conditions are favorable. To ensure and optimum milling process, it is recommended to dry the paddy rice to a moisture content (MC) of around 14% wet basis [1, 2]. During the dry season, the grain temperature can reach up to 55 \({}^{\circ}\)C due to the direct exposure of the paddy rice to high solar radiation. Under these conditions, the paddy rice can reach a very low MC (<10% wet basis) [3, 4], which may result in fissured grains after milling, thus decreasing its market value. However, during the rainy season, the lower radiation and extended rainy periods may lead to a prolonged drying process. This delay in drying increases the risk of paddy rice deterioration. This deterioration of paddy rice can be observed in the milled rice. The sun drying of paddy rice is a complex process, which involves the removal of water through direct exposure of the crop to solar radiation. Heat is transferred from the surroundings and from the sun to the exposed crop surface. Raising the crop temperature helps to diffuse water vapor from the interior of the kernel to the surface. Hence, mathematical models have been developed to understand and predict the sun drying process of different agricultural products. [PERSON] and [PERSON] [5] proposed the calculation of the convective heat transfer coefficient (h\({}_{\text{c}}\)), followed by the generation of a mathematical model for the estimation of the crop temperature (T\({}_{\text{crop}}\)) and moisture evaporated (m\({}_{\text{ev}}\)). To simplify the model's solution, a steady state condition is commonly used, and indeed several authors follow this methodology [6; 7; 8; 9]. Even though meteorological conditions have a strong influence on the solar radiation incident on the crop surface, constant radiative properties, such as the reflectivity and absorptivity of the crop, are commonly assumed during the modeling of the sun drying of medicinal plants, cereals, vegetables, and fruits. A fraction of the incident solar radiation is partly absorbed and scattered by the atmosphere and the earth's different surfaces. Here, the surface reflection (albedo) is the ratio of the reflected solar radiation to the incoming solar radiation [10; 11]. Albedo plays a crucial role in the fraction of solar energy absorbed and is a fundamental parameter applied in different research fields, including agriculture, climatology, forestry, building energy, and photovoltaics [12; 13; 14; 15]. Previous studies have explored the parametrization of albedo, and most of them reported a close relationship with the solar zenith angle [16; 17; 18; 19]. The parametrization of albedo has commonly been used for the improvement of the accuracy of models such as land, weather, and climate forecast models [20]. In rice production, several studies have assessed albedo measurements during the whole cropping period, and some even monitored it up until the harvesting stage. The values ranged from 0.09 to 0.25 [10; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. This information is commonly used for the estimation of surface energy, water budget, and potential cropping yield. However, limited research on albedo in the postharvest processes was found, particularly in the sun drying of paddy rice, where solar energy plays a critical role in the success of drying. [PERSON] and [PERSON] [31] suggest albedo values depending on the surface color of the product: dark-colored materials have an albedo value of 0.10, while gray, red, and green materials have a value about 0.30, and light-colored materials have a surface value of about 0.50. For brown rice, [PERSON] [21] and [PERSON] [32] proposed an albedo value of 0.16.