Texas Water Code - WATER § 58.313. Interest and Collection Fees

(a) All assessments imposed under Section 58.305(a) shall bear interest from the date payment is due at the rate of 15 percent a year.  Assessments not paid by the first day of the month following the date payment is due are delinquent, and a penalty of up to 15 percent of the amount of the past-due assessment shall be added to the amount due.

(b) If suit is filed to foreclose a lien on crops or if a delinquent assessment is collected by an attorney before or after suit, an additional amount of 15 percent on the unpaid assessment, penalty, and interest shall be added as collection or attorney's fees.