## 1 Introduction
### Rice Growing Stages and Key Characteristics for SAR Based Detection  
Rice in tropical and subtropical Asia is mainly cultivated in irrigated or lowland rainfed conditions. Rice varieties range in duration from 90 to more than 150 days and with three main crop stages: vegetative (from germination to panicle initiation, from 45 to 100 days), reproductive (from panicle initiation to flowering, around 35 days) and maturity (from flowering to mature grain, around 30 days) (Figure 1). The following aspects contribute to the change in space occupied by the rice plants within a three-dimensional space: (1) appearances and growth of leaves from the main stem (culm) and tillers; (2) stem development and elongation; (3) tillering, defined as the production of stems from rice plants; (4) leaf senescence; and (5) panicle and grain development. Rice leaves appear approximately every four days during the vegetative phase and every seven days during the reproductive phase [26]. Tillering begins at around the three- to four-leaf stage or approximately 10 days after emergence. Leaf production progresses from the bottom to the top part of the canopy, with the later-appearing leaf blades tending to be greater in length. Vertical growth of rice plants in the early growth stages (early tillering phase) is mainly contributed to by the development of new nodes along the main stem. At the later stage of the tillering phase, towards the panicle initiation stage, stem elongation begins and contributes to a rapid increase in the vertical expansion of the rice canopy. Leaf senescence begins at around heading and contributes to the declining leaf area index. In most cases, the rice leaf area index peaks at around heading. At the heading stage, panicle extension occurs when panicle tips emerge from the base of the last leaf, which is known as the flag leaf. Subsequently, panicles will grow in length and volume and will bear developing grains. By the maturity stage, individual panicles for typical modern, short- to medium-duration rice varieties occupy about 20% of the space in the top layer of the canopy. However, when arranged along with panicles from all rice tillers, panicles and grains together occupy about 30% of the top layer of the canopy. For typical plant heights of around 100 cm, panicle and grain occupy about 20 cm of the top layer in the canopy. During the ripening stage of grain development, lasting 25-35 days, the overall rice plant's water content decreases [14]. Apart from biology, agronomic management influences biophysical changes in rice plants within the three-dimensional space they occupy, mainly because of the different spacing between rice plants. In the transplanted system, rice plants are grown in a seedbed for about 20 days, and then, the plants are transplanted in a hill configuration at about two plants per hill; a common spacing is 20 \(\times\) 20 cm, but this can vary. Prior to transplanting, the rice field is flooded with water at depths ranging from 2 to 15 cm [14]. This deliberate agronomic flooding is a key element of most remote-sensing rice detection algorithms [13]. After transplanting, the recommended practice is to keep the water level at about 3 cm and gradually increase it to 5-10 cm with increasing plant height. With rice established using direct seeding, rice seeds are sown at higher density per unit area directly in wet soil or soil with a water level of 2-5 cm [14]. Because of higher population density under direct seeding, the production of tillers is suppressed [26], and thus, the increase in leaf volume within the canopy is mostly contributed to by leaf appearance and growth from the main culms. The water level with the direct-seeding system is kept low until the plants are large enough to withstand shallow flooding at the three- to four-leaf stage. Keeping the water level low during the first 10 days after transplanting or 21 days of direct seeding is recommended for golden snail management, whereas, until about the same time, keeping ample water level in the field is important to suppress weeds.