filtering jqgrid based on user input

everything is working fine with my jqgrid except a small issue. i have defined postData below:

 $(document).ready(function() {

$("#ctl00_ContentPlaceHolder2_drpUSite").change(function() {
    site = ($("#ctl00_ContentPlaceHolder2_drpUSite").val());
    loadusergrid();
});
var usrparams = new Object();
var site = ($("#ctl00_ContentPlaceHolder2_drpUSite").val());
//----grid code---------
$("#users").jqGrid({
    prmNames: {
        _search: "isSearch",
        nd: null,
        rows: "numRows",
        page: "page",
        sort: "sortField",
        order: "sortOrder"
    },
    // add by default to avoid webmethod parameter conflicts
    postData: { searchString: '', searchField: '', searchOper: '', sites: site },
    datatype: function(postdata) {
        mtype: "GET",
        $.ajax({
            url: 'Users.aspx/GetUsers',
            type: "POST",
            contentType: "application/json; charset=utf-8",
            data: JSON.stringify(postdata),
            dataType: "json",
            success: function(data, st) {
                if (st == "success") {
                    var grid = $("#users")[0];
                    var m = JSON.parse(data.d);
                    grid.addJSONData(m);
                }
            },
            error: function() {
                alert("Loading Failed!");
            }
        });
    },
    // this is what jqGrid is looking for in json callback
    jsonReader: {
        root: "rows",
        page: "page",
        total: "total",
        records: "records",
        cell: "cell",
        id: "login",
        repeatitems: true
    },
    colNames: ['Login', 'First Name', '开发者_高级运维Last Name', 'Email', 'Site', 'Role', 'Room', 'UnitID', 'Supervisor', 'Super'],
    colModel: [
    { name: 'login', index: 'login', width: 20 },
    { name: 'fname', index: 'fname', width: 20, hidden: true },
    { name: 'lname', index: 'lname', width: 60, align: "center", sortable: true, searchoptions: { sopt: ['eq', 'ne']} },
    { name: 'email', index: 'email', width: 20, align: "center", sortable: false },
    { name: 'site', index: 'site', width: 50, align: "center", sortable: true, searchoptions: { sopt: ['eq', 'ne']} },
    { name: 'role', index: 'role', width: 15, align: "center", sortable: true, searchoptions: { sopt: ['eq', 'ne']} },
    { name: 'room', index: 'room', width: 30, align: "center", sortable: true },
    { name: 'unitid', index: 'unitid', width: 10, align: "center", sortable: false },
    { name: 'super', index: 'super', width: 20 },

    { name: 'supername', index: 'supername', width: 10, align: "center", sortable: false },
],

    pager: "#pageusers",
    viewrecords: true,
    caption: "Registered Users",
    imgpath: 'themes/steel/images',
    rowNum: 20,
    rowList: [10, 20, 30, 40, 50],
    sortname: "pname",
    sortorder: "desc",
    showpage: true,
    gridModel: true, gridToolbar: true,
    onSelectRow: function(id) {
        var ret = jQuery("#users").getRowData(id);
        accpara.id = ret.id;
        accpara.pname = ret.pname;
        accpara.pid = ret.pid;
        accpara.bld = ret.bld;
        accpara.cname = ret.cname;
        accpara.amt = ret.amt;
        accpara.status = ret.status;
        accpara.notes = ret.notes;
        accpara.lname = ret.lname;
    }
});
jQuery("#users").navGrid('#pageusers', { view: false, del: false, add: false, edit: false },
 {}, // default settings for edit
 {}, // default settings for add
 {}, // delete
 {closeOnEscape: true, multipleSearch: true,
 closeAfterSearch: true
 }, // search options
 {}
 );
$("#users").setGridWidth(1300, true);
$("#users").setGridHeight(500, true);
jQuery("#users").jqGrid('filterToolbar');
//----grid code ends here



   function loadusergrid() {
       $("#users").setGridParam({ page: 1 }, { pgbuttons: true }, { pginput: true }, { postData: { "site": site} }).trigger("reloadGrid");
    }
});

when page loads for the 1st time, this works..

now i have 4 drop-downs which filter users. i have written a function which reloads the grid when the dropdown is changed, but it isnt working.. what am i doing wrong here?? when i enable postback for the dropdowns, i get the filtered result. i want to avoid postbacks on my page :). right now i have added just the site dropdown as the filter. once this starts working ill add the remaining 3.

firebug shows the ajax call is fired successfully but with an empty sitename. please note that the site dropdown cntains an empty value when page is loaded for the 1st time.

thanks in advance

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$("#mygrid").setPostData({parameters}); does the trick