How much agricultural Gaza land does Israel's buffer zone take?

One of the oft-repeated claims about Gaza is that about 30% of its  farmland is eaten up by Israel's buffer zone. This claim has been made by many, including the UN, as in this quote from 2009:

 "Bear in mind that 30% of Gaza's most productive land is within that buffer zone."

More often, the figure given is as a percentage of "arable land" - for example, this UN document says that "The  area  inside  the  Buffer  Zone  along  the northern  and  eastern  borders  with  Israel contains  nearly  a  third  (29%)  of  the  Gaza Strip's  arable  land,  and  is  inaccessible  to farmers."

However, a recent conference in Gaza by the hardly impartial "Palestinian International Campaign To End The Siege On Gaza" claims that Israel's buffer zone takes up 22.5 km2, or 6.25% of Gaza's total land, and that it takes up 20% of Gaza's arable land.

Both these claims are absurd.

The second, smaller claim is easy to demolish. According to the CIA World Factbook, Gaza has 29% arable land, or 104.4 km2. Gaza's total area is 360 km2. If the buffer zone takes up 22.5 km2 - a debatable point itself - then that means that 100% of Israel's buffer zone is arable land, and zero percent is desert or unusable.

A quick glance at Google Satellite images shows that while there are many strips of green land at the border,

much of the southern half of Gaza has no such usable land (here is near the Gaza airport:)

Similarly, much of the northern border with Israel does not appear to be usable for agriculture, as parts are urban.

So even the claim that Israel takes up 20% of the arable land is nonsense, according to their own figures.

Now, how about the claim that Israel's buffer zone is 22.5 km2 to begin with?

Israel officially sets the buffer zone at 300 meters. Israel's border with Gaza is 51 km, so that multiplied by the 300 meters comes out to 15.3 km2, not 22.5. This is a little harder to argue because Israel's attackers claim that the buffer zone in reality extends as much as a full kilometer into Gaza at certain points, but without bringing too much hard evidence for this claim. Without seeing a map of the supposed real buffer zone I cannot check those claims.

There is another important fact to point out, however: Most people don't know what "arable land" means, and think that it is identical with "farmland." However, that is not the definition at all. The UN defines two important terms:

Arable land is the land under temporary crops, temporary meadows for mowing or pasture, land under market and kitchen gardens and land temporarily fallow (for less than five years).

Land under permanent crops is the land cultivated with crops that occupy the land for long periods and need not be replanted after each harvest.

For the purposes of calculating the amount of land available for agriculture, both those types of land must be included.

Guess what? While Gaza has 104 km2 of arable land, it also has over 75 km2 of land under permanent crops!

So, contrary to the UN quote above, Israel isn't taking up 30% of Gaza's "most productive land". It is, at most, using up loser to 12% - even assuming that 100% of that buffer zone land is arable. In fact, the percentage is probably one third less, from eyeballing the map, or perhaps 9% assuming the figure of 22.5 km2 is accurate.

If Israel's figures of 15.3 km2 is closer to the truth, again assuming that one third of that land is unusable, then Israel's buffer zone might be taking up closer to 5% of Gaza's potential and real agricultural land.

There is a big gap between the 30% quoted by the UN above and the reality of 5%-10%.