A. Matev, R. Petrova Abstract. The aim of this work is to establish parameters of relationship “yield-irrigation depth” for sunflower (grown in the region of Plovdiv), using different equations, as follows: y = ax2 + bx + c; y = y + 2 (1–y ) x – (1–y )x2; Y = 1 – (1 –Y )(1 – x)n. The