var field = document.getElementById("ctl00_Header1_Search1_KeywordField");

function clearField() {
    if (field.value === "SEARCH") {
        field.value = "";
    }
}
function populateField() {
    if (field.value === "") {
        field.value = "SEARCH";
    }
}

field.onfocus = clearField;
field.onblur = populateField;