If the assessments in any special assessment roll prove insufficient for any reason, including the noncollection of the assessments, to pay for the improvement for which they were made or to pay the principal and interest on the bonds issued in anticipation of the collection of the assessments, then the irrigation board shall make additional pro rata assessments to supply the deficiency, but the total amount assessed against any parcel of land shall not exceed the value of the benefits received from the improvement.