Because they follow the paddy fields - i.e. the water paddocks. They aren't ploughed like dry fields.
A paddy field is a flooded parcel of arable land used for growing rice and other semiaquatic crops. Rice can also be grown in dry-fields, but from the twentieth century paddy field agriculture became the dominant form of growing rice.
from water researves irrigation
The word paddy is generally used for rice or unmilled rice. In this case the answer is a bit lame: paddy is used as a food crop. Paddy could also refer to paddy fields; flooded and sometimes terraced rice fields often seen in South East Asia. Paddy's are used to secure water for the always thirsty rice plants. The practice of leaving the fields submerged during the growing season keeps away many weeds (most plants can't root under water) as well as some pest animal like rodents and certain types of insects. Although wet rice cultivation it's the mostly used method it's also possible to grow rice on dry land. Because of the high water needs of most rice varieties this is only possible using irrigation or in regions with high and reliable rainfall.
Floods could damage crops and soil, leading to significant losses for farmers. Droughts could reduce water supply for irrigation, stunting crop growth and reducing yields. Farmers built irrigation systems such as canals, wells, and dams to bring water to their fields during dry spells or invested in drought-resistant crops to adapt to the changing conditions.
Sumerian farmers utilized an advanced irrigation system to water their fields during the dry summer months. They constructed canals and ditches to channel water from the nearby rivers, particularly the Tigris and Euphrates, directly to their fields. This method allowed them to control water flow and ensure that their barley crops received the necessary moisture for growth. Additionally, they may have used techniques such as basin irrigation to maximize water retention in their fields.
Irrigation
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Farmers get their water by the irrigation. (called the Central Valley Project)
Dry Farming
Dry Farming
Around autumn when the fields are dry.