禾 depicts a stalk of grain bending under its ripening ears and serves as the indexing radical here. The link to sparseness comes from agriculture: thinly planted seedlings are 稀 - too few stalks per row. Compare 稠 (dense), 移 (transplant), 秧 (seedling).
希 (xī) supplies the sound and a strong semantic echo - 希 itself already carries the sense of rare or scarce (希少). 稀 specialises this into the sparseness of crops, then generalises back out to thin, watery, rare. Tone unchanged.
