R: getting data from website, method POST, dropdown menu options change - html

Question

I'm trying to use R to extract data from a website where I have to select information from 5 dropdown menus and then click on an export or consult button (http://200.20.53.7/dadosaguaweb/default.aspx). I found this excellent thread: Getting data in R as dataframe from web source, but it didn't answer my question because of some differences:
1) The website's form's method is Post, not Get;
I tried using the RHTMLForms package together with RCurl, in a way that would work for Post or Get. Namely:
baseURL <- "http://200.20.53.7/dadosaguaweb/default.aspx"
forms<-getHTMLFormDescription(baseURL)
form1<-forms$form1
dadosAgua<-createFunction(form1)
dadosDef<-dadosAgua(75,"PS0421",1979,2015,6309)
2) The website is one of those where the list of options for the second dropdown menu changes according to what you selected for the first one and so on. Therefore, when I set the first input parameter to "75", it does not accept the second one as "PS0421" because that option is not available when the first parameter is at its default value.
So, I tried a step-by-step approach, changing one parameter at a time, like this:
baseURL <- "http://200.20.53.7/dadosaguaweb/default.aspx"
forms1 <- getHTMLFormDescription(baseURL)
form1 <- forms1$form1
dadosAgua1 <- createFunction(form1)
dadosDef1 <- dadosAgua1(75)
forms2 <- getHTMLFormDescription(dadosDef1)
form2 <- forms2$form1
dadosAgua2 <- createFunction(form2)
dadosDef2 <- dadosAgua2(75,"PS0421")
And I get the error message:
Error in function (type, msg, asError = TRUE) : Empty reply from server
Now I'm completely stuck.

Answer 1

I think what you're trying to do is navigation scripting, i.e. getting code to interact with a webpage. It may be complicated to do that programatically, because in order for the fields in the form to change in response to what you click, you have to actually be on a web-browser.
An alternative might be for you to use a tool that can do that for you, like CasperJS, which uses a headless browser, so the page fields can change based on behaviour you script. I don't know how comfortable you are with Javascript, and I don't know of any R packages that can do what casperjs does, so I can't recommend anything else.
Edit:
Take a look at RSelenium